Yes - I know of people running SOnar 7 and Cubase 4.1 on PC platforms with Vista 64 because they are lucky to have the drivers and they are delighted to be able to access 8 GB of Ram and have their quads/octacores working well. Edirol are an exception to the rule unfortunately. M-Audio are only just beta testing, many other platforms have sporadic support for some interfaces, some are using it as an excuse to drop solid items from their product line.
MOst pro studios I know of only run Pro Tools or Logic
More home recordists are using Mac too, given that it can run these programs, and when you get Logic STudio 8 at $499, many people are leaving windows and taking the plunge for Mac. Cubase and Sonar are 300 dollars more expensive, and you get a lot less in the box (esp. Cubase). Logic Studio, with Soundtrack Pro for audio editing, waveburner for burning red book compliant CDs, plus it's myriad of instruments and FX and newly designed interface just represents amazing value. Yes, it is a loss leader to bring people to Mac and to put pressure on Digital Performer, Cubase and PTLE on Mac, but still, exceptional value.
Anyway, the point if, I will never know the exact truth of Microsoft and the 3rd party support, all I know is that it is all over the place and I don't want to have to buy new audio interfaces and controllers just to use Vista.
Also, my personal experience of Vista (32 bit Vista Business) is terrible, My wife has it and I'm forever having to try and fix it and some things will still not work right, even after clean reinstallations - I'm talking about elements of Microsoft Office here too, not just 3rd party applications, but every day the error list and incompatibililty list and 'this application has a performance issue related to a corrupt/missing file or driver which will slow performance' (on MS Outlook 2007) is just so annoying. I think MIXED is the best way to describe Vista performance right now, if you have it configured nicely and have the drivers, then it is fine but many do not and are having problems and most of these issues did not exist in their XP installations.