My Windows 7 VM was created under Fusion 3 and upgraded to Fusion 4, like yours.
I can't think of any other subtleties. I assume you've already made sure all your video drivers are up to date, on both the host and the guest.
A thought - on mine, both monitors are identical and are set to the same resolution, pixel count, etc., on both the host and the guest OS's. True for you too?
The only other thing I might try would be, in the Win7 display settings, try a temporary setting that is different from "Duplicate these Displays". You might also try letting Windows fully establish your temporary setting (don't click Revert), then go back into Display Settings again, afterwards, and set it to the desired "Extend these Displays".
Let me know how it goes.
No one from VMware has contacted me about this issue. Come to think of it, they've never contacted me about any issue!!
Thanks