A new attack has been discovered against AES.

"Abstract. In this paper we present two related-key attacks on the full
AES. For AES-256 we show the rst key recovery attack that works
for all the keys and has complexity 2119, while the recent attack by
Biryukov-Khovratovich-Nikolic works for a weak key class and has higher
complexity. The second attack is the rst cryptanalysis of the full AES-
192. Both our attacks are boomerang attacks, which are based on the
recent idea of nding local collisions in block ciphers and enhanced with
the boomerang switching techniques to gain free rounds in the middle."

and here's the important part

"both our attacks are still mainly of theoretical
interest and do not present a threat to practical applications using AES." -

Bruce Schneier has also responded to this research on his blog.

Quoting Bruce "Agreed, while this attack is better than brute force -- and some cryptographers will describe the algorithm as "broken" because of it -- it is still far, far beyond our capabilities of computation. The attack is, and probably forever will be, theoretical."

Attack whitepaper: https://cryptolux.uni.lu/mediawiki/uploads/1/1a/Aes-192-256.pdf
Bruce's Take: http://www.schneier.com/blog/archives/2009/07/new_attack_on_a.html
Coverage @ TheRegister: http://www.theregister.co.uk/2009/07/02/aes_crypto_attacks/