The Cray XE6 accepts Opteron 6200-series chips, all of which support the AES Instruction Set. In AES-NI Performance Analyzed (which is Intel, not AMD), Patrick Schmid and Achim Roos found that AES NI has a throughput of 3.5 cycles per byte. If we extrapolate that to the 128-bit (16-byte) AES-256 block, we get 56 cycles per AES-256 operation. The Opteron 6282 SE works at 3.1 GHz in All Turbo mode. Assuming that the Opteron 6282 SEs' AES Instruction Set has the same performance as Intel AES New Instructions, an Opteron 6282 SE core might do ~55,357,142.857143 AES-256 operations per second. Therefore, a Cray XE6 with one million Opteron 6282 SE cores might do ~55,357,142,857,143 AES-256 operations per second. The figure does not take into account neccessary I/O operations.
Therefore, to brute-force an AES-256-ECB encryption key in a known-plaintext attack, using all possible combinations, on a Cray XE6 with one million Opteron 6282 SE cores, it would take up to ~66,282,862,563,751,221,625,826,507,369,649,000,000,000,000,000,000,000,000 years to complete the known-plaintext attack. However, if the encryption key is derived from a 10-character pass phrase consisting only of English lowercase letters a-z (26 ^ 10 = 141,167,095,653,376 possible combinations), it would take that same Cray XE6 up to ~2.55 seconds to complete a non-dictionary known-plaintext attack. If the encryption key is derived from a 10-character pass phrase, possibly consisting of English lowercase letters, English uppercase letters, numbers, and 22 other characters (84 ^ 10 = 17,490,122,876,598,091,776 possible combinations), it would take that same Cray XE6 up to ~87.76 hours to complete a non-dictionary known-plaintext attack.
I calculated the duration of non-dictionary known-plaintext attacks on AES-256-ECB with one million cores @ 3.1 GHz using Intel AES New Instructions. You can see it here. I guess it would still be useful to keep something with one million cores @ 3.1 GHz with Intel AES NI around to brute force up-to-10-character pass phrases in a known-plaintext attack.