There are LOTS of programs that don't work under vista, but they aren't the major programs. The reason is obvious if you're a software developer at the other end. First off, corporate users are still on XP so your client base isn't screaming at you to get this work done. Secondly, it's thankless work. Right now I'm working on a port to Vista of some ActiveX stuff that runs under the browser, and getting everything to interact smoothly in the browser with UAC (their new protection mode) is an absolute pain in the rear. And of course, if you interact with 3rd party libraries they may need porting, etc. etc.
My favorite issue so far is that certain controls built with Microsoft's Visual C++ 2003 will CRASH under vista due to Vista's new protection mode (DEP). Microsoft's recommended fix: recompile the controls under VC++ 2005. Which of course, requires a few more fixes since VC++2005 has increased standards compliance. Nice....