It's major rumor -month for Apple products, and the latest that's hitting the internets suggests that there's a 15-inch version of the super-skinny MacBook Air on the way. This rumor even gets a little credibility from its source--a Taiwanese blog that leaked out the aluminum unibody MacBook construction.
Okay, so the source--Apple.pro--also got it wrong by leaking out fake images of the MacBook, but hey. The new rumor says little more than that they've heard (presumably from manufacturer contacts, a little like the unibody leak) that Apple's working on a 15-inch MacBook Air.
And this rumor actually makes some sense. The original Air seems to have created its own small class of super-portable notebooks, with Dell  and a few  others  coming up with machines in the same general format--Apple would we foolish to ignore the opportunity of expanding this class with another machine.
Apple's recently updated  the Air with beefier CPU and graphics chips, and the larger body offered by a 15-inch machine would allow the Air to have even more serious performance-boosting circuitry inside. A super-portable 15-inch display machine would be loved by people who need the screen real-estate (I'm thinking designers, possibly) and would benefit from the ultra-portability.
And with the recent 17-inch MacBook Pro, Apple's been bold enough to take the non-removable battery  idea from the Air and use it in a bigger machine. It's perfectly possible that Apple's further developed this battery design, and has one ready for a 15-inch machine. And a bigger Air would allow for a significantly bigger battery, removing the original 13-inch machine's biggest flaw: poor battery life.
Assuming it didn't cost a whack load more than the already-expensive 13-incher, I, for one, would buy one like a shot.