Taking a momentary breather from the joys of Windows Presentation Foundation, I wanted to point to this excellent MSDN article on testing applications for compatibility with Windows Vista.
Some of you may remember that at the PDC in 2003 one of the first "Longhorn" demos we did was of a twenty-year old DOS application (Visicalc, if memory serves correctly), so it should be clear that maintaining compatibility is important to us. Nevertheless, the introduction of security features such as User Account Control, IE Protected Mode and User Interface Privilege Isolation are definitely the kind of things that can break unwary applications, let alone other obvious changes such as updated version numbers and the need to support 64-bit editions of Windows that are becoming increasingly prevalent. This white paper has a suggestion for a thirty minute compatibility check, as well as overviews of each of the major changes that could potentially break applications and links to further information. Well worth a quick read if you're in the business of shipping quality software.