Well a group of researchers, including Adi Shamir (whose last name puts the S in the RSA algorithm) and Alex Biryukov, Orr Dunkelman, Nathan Keller and Dmitry Khovratovich.
The paper, which details the attack, has yet to be made public. Brice Schneier has read the paper, and provided a peek on his Web site:
In this paper we describe several attacks which can break with practical complexity variants of AES-256 whose number of rounds are comparable to that of AES-128. One of our attacks uses only two related keys and 239 time to recover the complete 256-bit key of a 9-round version of AES-256 (the best previous attack on this variant required 4 related keys and 2120 time). Another attack can break a 10 round version of AES-256 in 245 time, but it uses a stronger type of related subkey attack (the best previous attack on this variant required 64 related keys and 2172 time).
The paper, detailing the attack technique, should be available within days. For now, Schneier is recommending people not use AES-256 and that AES-128 "provides more than enough security margin for the foreseeable future."
Let's hope so.