I moved a guest RHEL 5 update 1 from ESX 3.0.1 to 3.5.1 tonight. It wouldn't boot. Well, it would start to boot, get to init and then shut itself off with no error messages on the guest or host.
Culprit: Host options had the OS still set to RHEL4 (from 3.0.1). Switching it to RHEL5 made the world turn again (after much worrying).