Here is an improved method, with plenty of screenshots, for
moving a VM from Parallels Desktop for Mac (build 1970) to the current beta
build of Fusion (build 36932). Like the previous method I posted,
this technique uses a free ($0) mode of VMware Converter. The main
improvement is that a helper machine is no longer required: you
simply install VMware Converter directly into the Parallels VM. It
is also no longer necessary to disable the source VM's Windows
Firewall, nor to disable the "simple file sharing" feature.

Remaining limitations of this technique:

• Like VMware Converter itself, the technique only works with Windows XP, 2003, 2000,
  or NT4 virtual machines. It doesn't work with Linux VMs, and it also doesn't
  work with Vista VMs.
  
• Downloading VMware Converter requires answering the usual
  annoying marketing questions.
  
• If your source VM was not licensed with a volume-license key, the
  clone in VMware Fusion might demand to be reactivated.
  

First, log into your source Parallels VM as Administrator or as a user with administrative privilege. Here's a screenshot of my VM after I logged in.

We need to notice something about Parallels VMs in Device Manager:

Notice that, underneath Computer, it says "Standard PC," not "Uniprocessor ACPI" or "Multiprocessor ACPI" like modern PCs. It seems that Parallels (at least build 1970) makes non-ACPI
machines. ACPI is the modern standard for PC software power
control. More on this later.

First we must enable Windows Sharing on our host Mac. Very
important!

It doesn't matter what network mode the source VM is in, as long as it can mount a Windows file share from the host. Confirm that now.

We are now ready to download VMware Converter into the source VM. Grab it from
http://www.vmware.com/download/converter/, and run the installer.

When you launch the installer, you'll see this screen:

Notice that text under the lightbulb. Don't let it send you astray! Use of Converter in the mode it describes requires buying an enterprise Converter license. So disregard the advice about booting from a VMware Converter CD unless you have such a license (in which case the instructions here won't apply to you).

Proceed to accept the license agreement, and do a typical (default) installation.

When you run VMware Converter for the first time, it will invite
you to type in a license code:

You don't need a license for what we are doing here. Just click
Continue in
Starter Mode.

Launch the Import Wizard.

The first step is to specify the source machine. Tell Converter that you
want to image a "physical computer." Yes, this is a lie. Do it anyway.

In the resulting screen, pick "This local machine".

When you click "This local machine," the Authentication boxes get grayed out. Converter assumes that you are running with administrative privilege.

Next Converter will query your source machine for what disks it has. My Parallels VM has only one disk, its C: drive.

That's the one I want to clone.

In the next screen, tell Converter that you want to create a "standalone" virtual machine.

Now Converter will ask you for a name for your new VM, as well as where to write the VMware virtual machine it'll create from your Parallels VM. Remember that

you enabled Windows file sharing
on your Mac? That's where you're going to write the new VM: on the host. Put in a "whack-whack" path (more formally called

a "UNC path") that points at
your Mac home directory, as shared by the Mac.

Converter will make a subdirectory in the UNC path you name with the same
name as the VM, and then put its .vmx file and its virtual disk in there.

Pick Workstation 5 as the destination VM type, even though Fusion is actually
more like Workstation 6.

Now you'll get a popup to supply your Mac login credentials.

You'll next be asked what kind of virtual disks to make for this virtual machine. You
may choose either "Allow virtual disk files to grow" or "Allocate all disk space
now for better performance."
Most users wouldn't notice the performance
difference of preallocated disks, but you might if you plan to do something
especially disk-I/O-intensive in this VM. If you, like me, are always running
out of disk space, "Allow virtual disk files to grow" will be a good choice.

You cannot uncheck the box marked "Split disks
into 2 GB chunks," but it is a good choice anyway.
Dividing disks into 2 GB chunks means that
they can be safely stored in a FAT file system, such as many Mac users
have on their external hard disks.

Next Converter will ask you to confirm the network configuration for the
new VM. Assuming you're using beta build 36932 of Fusion, choose bridged or NAT, because host-only is not yet supported in this build.

The next screen will ask you whether you want to customize the VM after it
reboots using sysprep, which is Microsoft's tool for

automatically changing the security ID and the computer name of a clone of a Windows image.) If you're just migrating this VM from Parallels to Fusion, and you don't plan to run the source and the destination at the same time (which would
require the purchase of an additional Windows license unless you have a volume
licensing agreement), there's no need to customize.

Notice that that screen does not offer you the chance to install VMware Tools in the new VM. It's just as well, since we need to put the brand-new Fusion version
of VMware Tools in after the conversion.

Finally you're at the last screen of the wizard. Converter summarizes what
it is about to do.

For me, the transfer only took a few minutes.
Here's what it looked like in progress:

When the transfer finishes, shut down your Parallels VM. (If you don't shut down
your Parallels VM before starting up the Fusion VM cloned from it, you'll be in
violation of your Microsoft license agreement unless
you have a volume license. Also, depending on your VMs' network configuration,
you might get a "duplicate computer name" or even a "duplicate IP address" warning.)

Now tell Fusion to browse to the newly created .vmx file and power it on! You'll
be asked what to do about the VM's unique identifier (UUID). Since you're migrating
this VM, not copying it, "Keep" is the right choice.

Parallels VMs always have a virtual floppy drive, because that's how they
get their Parallels Tools into VMs. This VM will be equipped with a virtual floppy
drive too. But VMware VMs have no need on their own for a floppy drive. So,
since your Mac has no physical floppy drive for Fusion to map your virtual floppy
drive to, you'll get a harmless error message.

You can always delete the virtual floppy drive from the VM later if you want.

There will be a long spasm of newly detected hardware; let it die down before proceeding.

The new VM works great, except it has no mouse. To get the mouse working,
install VMware Tools (pull down the Virtual Machine menu and choose Install
VMware Tools). Regrettably, because the mouse won't work until the VMware Tools
installation is complete, you must work through the installer only the
keyboard.

You can use ALT-TAB in case the desired window loses mouse focus, ALT-F4 to get the "Found New Hardware Wizard" out of the way, and underlined letters in buttons to click them. Because the VMware Tools drives in this beta release of Fusion are not yet signed, you'll be pressing C for "Continue Anyway" a lot. Sorry.

Install all the parts of VMware Tools, even those you don't think you need right
now. After the installer reboots your VM, it's ready for use.

Remember that the source VM was non-ACPI. The new Fusion VM will also be non-ACPI,
unlike all VMs built natively on recent VMware products. What is the impact of this?

The user interface doesn't exit when Windows shuts down.
You must click the black power-off button to shut down, or else
add this line to your new VM's .vmx file:

gui.exitonCLIHLT = "TRUE"

(It's best to use a smart text editor like BBEdit to do this; TextEdit can mess
up a .vmx file if you cut-and-paste in the wrong way.)

Now your VM will shut down cleanly.

This VM also won't be able to do high-speed USB 2.0, because it is effectively
a Workstation 5 VM, and high-speed USB 2.0 is a Workstation 6 and Fusion feature.
If you want to use your iSight camera or a USB 2.0 disk in your VM,
add this line to your new VM's .vmx file:

ehci.present = "TRUE"

Enjoy! Please post any comments or feedback on this procedure.