Came across this bootup process of a MacBook Pro going from Mac OS X into XP. Seems pretty seamless, even if XP's boot process just looks plain wrong on a Mac :-) Using FAT32, you can even drag'n'drop between 'doze and Mac (but are limited to 32Gb for the largest partition size). Don't expect that NTFS writable drivers are around any time soon; Linux has had read-only support but no write for ages either.
Enjoy the show.
In a parallel development, you can even get hold of a virtualisation server that allows you to run Mac OS X and any other x86 box (Windows, Linux) at the same time. You can find out about it and for the time being, download a free beta that you can use to evaluate it. Pricing is rumoured to be set at $50 for when the full version is released.