Shoutbox

Apple's move to x86 won't affect the way Messenger for Mac is implemented. If an API in the Mac Messenger was in demand, it would have been put in already.

Not all software on Linux is open source. You can play Unreal Tournament 2004 on Linux, and that's closed source. In fact, there's nothing at all stopping people using closed-source programs on an open-source OS, same as there's nothing stopping you running open-source apps on Windows, a closed-source OS.
Then you have Mac OS X, based on FreeBSD. FreeBSD is open-source, but the OS X specific additions are closed source.

It all depends on whether you want to release the source or not.

Edit: I almost forgot, use Wine on Linux, and you can run Windows programs.