A new attack improves significantly on previous attacks against AES-256, see: http://schneier.com/crypto-gram-0908.html#8. This doesn't mean that AES-256 is broken yet, but the surprising bit here is that AES-128 is not susceptible to this particular attack. Don't panic if you are using AES-256 and read Bruce Schneier's commentary carefully - for example, note that the attack is against a 10 round AES-256, while standard implementations use 14 rounds.

It's interesting to see that the strength of AES-256 is eroded by new attacks, but AES-128 doesn't suffer from them yet. It's another example that larger (or more expensive) isn't necessarily also better.