Just a friendly advice for those who can afford to go both ways:
I was "stuck" with Windows during a one week trip while I was trying to mod two iPhones. It was a nightmare. Although the Windows applications are pretty advanced right now (iDemocracy, iBrickr, etc.) and there are pretty good instructions available, using OSX and the well known iNdependence, iFuntastic, command line, etc. makes iPhone modding/hacking a piece of cake.
Don't get me wrong, I'm using OSX, Windows and Linux, I'm not a Windows hateboy or an Apple fanboy but for iPhone mods/hacking, OSX works like a charm and I can highly recommend using OSX for this purpose.
So if you were ever curious about OSX (being a Windows or Linux user), this may be the best time to buy a MacMini or even a Macbook to start with it.