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.
Superior Windows drivers, superior Linux drivers, 5-6x faster per watt on Bitcoin, same game or better game performance per dollar (69xx really screws over Geforce 5xx on games, especially in Crossfire vs SLI or games that use a lot of tess), plus AMD is the primary financial backer of a completely open source modern GPU driver stack and Nvidia is only unofficially threatening to sue over the same thing?
Nvidia is the one that needs to get their act together.