The word is out directly from Intuit:
Backward Compatibility. Only QuickBooks 2007 will run on the Vista Operating System, as prior versions of QuickBooks were developed on pre-Vista technology and will not run properly on Vista. We know as a practical matter that many of you are supporting clients on previous versions of QuickBooks. For now the bottom line is that a user who upgrades to Vista will need to run QuickBooks 2007.