In other words: it lets you to flawlessly install Windows XP SP2 (be it the Home or Professional edition) in one of the new Intel-based Macs. Beta versions are now available for downloaded and the final version will be included in Mac OS X 10.5 Leopard.
Great, Apple. I do not have an excuse any more to not replace my desktop machine with a Macintosh. In fact, I'm dying to switch but I'll resist for some more time ;-)
Edit (April 7th, 20:57): Dirk Olbertz has installed Ubuntu on his iMac using Boot Camp (the link is in German). So this seems not to be restricted to Windows.