Well, I have to work on Windows, Mac and Linux. Not just using it, but really working on it, i.e. I have to manage to make programs compile, get used to the API and so on. If something doesn't work or if I don't know how it has to be done, I have to search the web or the documentation until I find a way to get it to work on that OS.
Personally I still prefer using XP.
Next would be Vista. And then a long time nothing.
But hey, everybody just use the OS which he likes most!