I wrote some time ago an article on which I described how to install Ubuntu on a Mac OS X via VirtualBox, and now I've installed a Windows XP on Ubuntu. So, it works pretty well, and the steps described in that article are 99% identical (with the exception of the first steps, where you select the Guest OS). The article can be found here: http://it-nthusiast.blogspot.com/2010/02/installing-ubuntu-on-macbook-via.html.
Some notes: You may have problems with the resolution, since the Guest OS doesn't see the Graphic Adapter. It will use a driver provided via Guest Additions. At times, Windows XP becomes quite unstable, but since now everything worked well (most of hardware worked out-of-the-box).
Below is screenshot which shows you my desktop, including my problem with my resolution.