Ars Technica agrees with Kennedy - developing for Windows and XP is not an either/or situation; developers can write software specifically for Windows XP, and still have those applications running just fine on Windows Vista. I have been using Vista since even before its official release, and apart from Ahead's Nero during Vista's early days, I have not encountered a single incompatible application. In other words, while analysts might conclude that 49% of developers are targeting Windows XP, compared to 8% who are targeting Vista, those 49% are still targeting Windows Vista, even if they don't know it. We are not talking OS9 or Mac OS X here.
The real question is, therefore, why aren't Windows developers leveraging the technologies in .Net and Vista? According to Kennedy, there are two reasons. Firstly, developers do not like targeting a platform that isn't widely available across the installed base. .Net doesn't ship with Windows XP, and the technologies in Windows Vista (Windows Presentation Foundation, Windows Communication Foundation, etc.) do not come with XP either, and Vista isn't yet as popular as Windows XP.
The second reason, according to Kennedy, relates to performance of the .Net framework.
Perhaps more interesting than the discussion surrounding Windows Vista are the figures concerning Linux: 13% of developers are targeting Linux, which can only be seen as a good thing for Linux, which makes it a good thing for the entire operating system industry. The more diversity, the better.