In case you live in a cave, Apple has announced the release of software called Boot Camp. This software allows any Intel Chip based MAC to run Windows XP (and likely Vista whenever it comes out) as a secondary OS on the machine. You would install Boot Camp and then when you want to shift from OS to OS, you would reboot while holding the option key (alt key for windows people) and then choose the OS you wanted to initialize.
If you have been developing for the web on a MAC over the last 10 years, then you know you need to view all of your work in a Windows environment several times before it goes live. Virtual PC has long been the standard for single computer developers with a MAC up until a few years ago when the price of PCs came down to the point that it was better to own a cheap PC than slow your system down with Virtual PC.
Well, here is something that all MAC users have been dreaming of ever since the Intel chip switch - Parallels.
EDIT: As always, John Gruber of daringfireball.net sums up the Apple move on Boot Camp in much more eloquent and in-depth way than I can.