Specifically Parallels, won't release a version that can run OSX under virtualization because they don't want their users to do anything illegal, or to open them up to legal issues. VMWare is wrestling with the same moral issue. Apparently Apple's ELUA specifically forbids virtualization.
I was just talking about this very thing yesterday. I know Apple is worried about running OSX on non Apple, hardware, but why not just work with them to make sure it doesn't? How cool (or ironic depending on your point of view) would it be to run OSX in a VM under Windows on Mac hardware? In fact I think if Parallels could work it so that they can virtualize either way while accessing your native partition, it would be good for their business and Apples.
Oh and if you want a laugh, read the last couple of sentences of that article where they speculate if it is even technically possible to run OSX under virtualization. Are they serious? Just search a few torrent sites guys, people have been running it under VMWare on non Apple hardware since the first Intel developer version came out! I guess I expect too much from ARS.