I know this is an old thread but, just to update for those new to ESXi, using ESXi 5 and the vSphere Client there are a handful of ways to move systems from one system to another that are all pretty straight-forward. The method I found (by which an experienced friend showed me because I didn't find one mention of this in the forums) is:
1. Power down the VM
2. Use the datastore browser to browse to where it is stored
3. Right-click> Move the entire folder to the desired datastore (presumably larger/faster/more free etc.)
4. Once moved right-click the VM> Remove from inventory
5. Use the datastore browser to go to the new storage location, navigate to the .vmx for the system. Right-click the .vmx> Add to inventory.
6. The machine will show back up in the list of VMs, but with an exclamation.
7. Click the VM, then the "Summary" tab. There will be a dialog that says something like "it looks like this system moved, did you move it or copy it or what?" with some radio buttons for Move/Copy/did not move. I chose "I moved it" each time and *walla* the system is moved from one datastore to another.
I'm sure most folks figure this out pretty easy but, being a n00b, none of this was intuitive to me. Hopefully I help someone out.
Enjoy!