I'm basically reiterating what others have said.
Mac for style, and for saying "I've got a mac"
Windows for gaming and (possibly) compatibility.
Linux for speed and cost.
Personally, I prefer both Linux and Mac to windows. I like Linux for the fact that it can be run from a USB stick (although you have to use a Windows computer to put it on the USB stick easily.)
I like Mac for the fact that it looks amazing and SHINY. Apple certainly put a lot of thought into how their OS looks.
I have a Linux desktop (which I am currently rebuilding) and a Dell Mini 10v. (1011, not 1012).
The downside to Linux is definitely the lack of easy-to-find support. There is plenty on the net, though - you just have to search for it.
The Dell originally had Windows XP on it, but now has (not completely legally) Mac OS X on it. It starts up in just over a minute (2 minutes faster than XP) and shuts down in just over 5 seconds.
My personal favourite OS is Mac, and I prefer Mac hardware too. Linux is amazing for a low price range though (I was given the laptop for my birthday), so I can see the benefits of that too. I jsut don't see why anyone other than gamers use Windows.