Hi, i have being looking for at short "howto" on this.
Since i didnt find any, i did a test enviroment myself. 
And why not share it with everybody.
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To do a hot vmotion from a 2.5 to a 3.0.1 here is the steps and the requirments.
You got some parts of your installation that must be a perfect match.
Regarding "Public / Shared" VMFS2 volumes.
Has no issue in my case. Did it with both Shared and Public Storage.
Notice points:
1. Network settings for VMotion
These are normal requirments for your Vmotion.
Naming of the vmotion networks on both 2.5 and 3.0.1
IP addresses must of course be the same subnet.
Remember to use the correct nics. ;o)
2. Network settings for the VM network
Naming of the Networks (on 2.5) and the Port Groups (on 3.0.1)
And those must reflect the same phycisal networks.
3. Datastores
The VMFS2 volumes containing the "VM 2.5" must be be visable to the ESX 3.0.1
The ESX 3.0.1 must have a other volume to put the "VM 2.5" to
Short step by step what it does. (Hasnt veryfied it yet.)
Vmotion RAM to new storage.
Register VM to the new ESX. (RAM only atm)
SnapShot created and placed on new Datastore.
Copy of the vmdk file from vmfs2 volumes to vmfs 3 volumes.
Commit Snapshot to the disks on VMFS 3.
Done!
And still just lost 1 ping.
I will verify it all later. And comments are welcome.