It was simultaneously developed for both platforms with Microsofts' help according to Jobs. So basically it can already run on X86 chips.
While Apple said windows would run (probably out of the box) on a X86 Apple they were dead set that OSX wouldn't run on anything other then a Mac though I'm sure that won't last long due to it being a Linux based OS. One annalist, in an article I was reading, said probably how Apple is going to do this is by having strict hardware requirements so that someone would have to know what they were doing when building a PC to get it to run OSX.
I personally think this is a mistake, I've been reading a lot of articles on this because I'm bored, and apple has a very good chance to pick up market share from Microsoft and especially Linux. Several of these articles were talking about how OSX, if were able to install on more common PCs, could single handedly cause the majority of Linux users to drop that OS and move to Apple which, as I'm sure you could deduce, would make Microsoft very happy.