Chris Pirillo has an intersting list of Ten Reasons Your Next PC will be from Apple. Some of them are the typical, run-of-the-mill reasons you hear everytime Mac vs PC comes up but some of them are really interesting, compelling reasons.  My favorite would be:

4. Knowledge. Don't let platform bigotry get in the way of understanding and appreciating good technology. We all are better for knowing more, and that's something you should look forward to doing – not avoiding at all costs. Bury the hatchet if you think that this still is the Mac you first saw in 1984.

This is the reason I have a Windows machine and a Linux machine to complement my main OS X laptop – it lets me be knowledgeable about the three OS’s and to be able to readily make the decision about which is best for a specific tasks, particularly because swapping OSs for a single task is straight-forward. Of course, if you’re a Mac user, this is an excellent reason why you should should put a Windows box within easy reach and preferably something like Linux, BSD or Solaris as well.