This is what I have said many times that apple should do, at least at a start. Unless they do some very good work with the hardware, OS X will probably be an option on most x86 computers, or at least newer ones. Apple is a huge winner in making this transition, ultimately making their life easier by lowering R&D costs and allowing them to focus more on software and other hardware items like the iPod.
The big loser is Microsoft. Apple can gain some serious market share, especially if they can offer their computers at a lower cost than they currently can, and this will hurt Microsoft quite a bit, especially if Longhorn isn't up to OS X standards, which I doubt it will be.
I don't think this hurts desktop linux as much as some people have thought. Desktop linux, at least in my view, is still a long way off for a lot of reasons. Usability is better, but its still not where Windows and certainly Apple are currently at, and they probably will getting better. This might also help server side linux as the Microsoft stranglehold gets more tedious.
And with this news, I wll just go ahead and say it. I will buy an x86 mac as soon as its offered. My objection to apple was their control over every aspect of all things Apple, and with this they are giving up a sizable chunk and becoming a sort of Dell-Microsoft hybrid company (in concept, not size). Thats fine, I didn't like the Intel-Microsoft-Dell supercompany idea much.