I seem to remember that with XP you could legitimately buy the upgrade version if you owned OS X. Is this still true (if it ever was) with Vista, as i know i will have to use Vista and would like to get it as cheap as possible.
If you own a Mac and a PC running XP that you recently bought with a free Vista upgrade coupon, then yes. Of course, the fact you have a Mac is irrelevant in that case.