I recently upgraded my host from 10.7.4 to 10.8.1, and Fusion from 4.1.3 to 5.0.1. I have two Mac OS X 10.7.4 guests I'd like to upgrade too, but the upgrade fails at the first reboot. I attach the USB key with the OS upgrade kit on it, click on Install OS X, wait about 10 minutes while it preloads things, then when it comes time to do the first reboot, the VM fails to boot, and drops into an "EFI Shell" command line prompt instead.
I've tried installing OS X 10.8.1 as a new guest, booting from the USB key, and it does the exact same thing.
The bootable USB thumb drive was created using the Lion DiskMaker 2 utility. To eliminate that as the culprit, I restored my guest from the pre-upgrade snapshot, logged into OS X 10.7.4 again, started up the app store, logged in and downloaded my already purchased Mountain Lion install app direct to my guest's virtual disk. Once that finished, I started the app, and the exact same thing happened again.
I guess that means Fusion 5.0.1 doesn't yet support OS X 10.8.1 guests.