How to run Mac OS X within Windows Virtual PC?
How do I virtualize Mac OS X on Windows XP host with VirtualBox?
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This is THE thread on the subject in the VBox forums.
Basically, because it's specifically prohibited by Apple (running it virtualised on non-Apple hardware), Sun can't work on making VBox properly compatible with it. This means no Guest Additions, at the very least.
However, it has been done on a basic level by users, albeit with various questionable hacks, and the result is unstable, slow and limited (and it's worse in VMWare, apparently).