Nothing is 100% compatible with any version of windows, there's always errors, patches, security holes, BSOD's, application lock-ups, etc. The rubber-stamp of a vendor that says "we support version X of our software on Windows XYZ..." doesn't mean as much as seeing it work over time.
I guess I approach this from the mind-set of the Unix/Linux world where a beta version of a given OS will work as well as a released version in terms of application compatibility. I ran a beta version of SuSE 10.something for a long time after the next version was released. It ran fine as did updated apps.
Perhaps when I beta tested Windows 2000 (er...NT 5) I should have kept it running in production until after XP came out. I wonder if it would have run everything that I could have thrown at it?? Never know I guess....