So far WB5.01 is functioning fine here on XP Pro /SP2 and many prominent company apps from 2002 through 2005:
Photoshop 7, Illustrator 10, CorelDRAW 12, Painter IX.5 (with the exception of resizing in conjunction with skins made for WB4.6), Acrobat 7, Bryce 5, Poser 5, etc.
The one exception seems to be ImageReady 7, which is not skinning the titlebar. I have not yet tested this with anything other than StealthOS (compact) - the alpha version.
Neil, I will create a post with some screenshots sometime this week illustrating the Painter IX.5 resize issue, and the ImageReady titlebar issue.