Are you sure that is right? OS X is not listed as a supported guest system, and I’m pretty sure Apple frowns on virtualizing it.
Apple frowns on a lot of things, but I know people do it. I doubt there’s anything wrong with it as long as you actually buy the OS. The only potential issue I can see if that with Snow Leopard being really new, it may take some time for VirtualBox to catch up to it.