Would it be possible to render the controls in XP and use the native ones in Vista?
Microsoft went down the "render the controls in legacy OS" route before with the VB runtime libraries several years ago. It was unpleasant. I remember ListView and TreeView columns looking and working like crap (redundant borders, column resizing behaviors
broken, etc). For a more recent example, I recall a Niner (Rossj, I think?) expressing displeasure with the pain that ugly WPF rendering in XP was causing him.
I think they're looking forward, not backward. Maybe we'll see Vista-style controls in the next version of the .NET framework a few years from now (Windows 7 timeframe). That version probably won't be required to support XP.