I just tried to open a Win7 Enterprise VM in Fusion 4.0.2 (the VM had already been ugraded). I repeatedly go the "Could not get snapshot information: Failed to lock the file. Module Snapshot power on failed." even after restarting VMWare Fusion.
I resolved this by quitting Fusion and then right clicking on my virtual machine and selecting "Show Package Contents". I then deleted the two folders called:
(they contained files of the same name).
I also deleted the cache files, though I think that step was unnecessary.
After restarting VMWare, the machine launched fine.
I know this solution has previously been described (I believe for version 2), but since that is rather dated now, I thought this meritted repeating.
PS I am running Lion (10.7.2) and an aging Macbook Pro.