On a more practical note than my last blog posting, I want to encourage everyone to check out the Windows Vista compatibility cookbook. Despite our best efforts, there will always be some breaking changes between releases of an operating system, particularly when security model changes such as UAC are introduced.
This excellent guide, recently updated for RC1 of Windows Vista, tells you everything to know about the changes that might break your application, from basic, simple-to-fix things (e.g. GetVersion() reports a major version number of 6) to more esoteric yet complex changes (the GINA extensible logon model has been replaced with a new credential provider model).
If you write Windows applications, I'd encourage you to download and read it - it's as simple as that!