The funny thing is that operating systems that are not supported by a huge company aren't very popular targets for hackers - chances are that XP will become more secure than ever. :P
(wouldn't bet on it, though)

I'd be more concerned with attacks that target Win 7 or 8, but where XP happens to have the same vulnerability. Which definitely could happen, MS is a particular fan of leaving vulnerabilities in until the next scheduled patch cycle. Regardless of whether they've already adequately tested it or not.

But, with the architecture changing so much with Win 8, I have no idea how likely that really is.