580 gets 127-130 mhash/sec at stock clocks with rpcminer-cuda on default settings under Windows 7 x64 (only one card so no SLI issues). 5850 gets somewhere around 240 mhash/sec or possibly more with the latest miners (I'm going off the wiki on this).
Welp. The 5850 is also $70 cheaper. Guess I'll suck it up and hope AMD got its act together.