I'll be sure to bump this thread up next year when we mass upgrade to Windows 7 at work where we have about 1500 workstations, majority of them being Windows based. Perhaps my opinion at that point may change from my previously stated one of 2k Pro.
Vista failed the test miserably when I did testing on it earlier this summer. I'm having high hope that Windows 7 won't. Sometimes towards Thanksgiving I hope to get a hold of a spare computer to do actual work on with it running Windows 7 and see how it does in a mass production environment.