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.
Not to get on your case, but I remember a time when the words "*nix" and "ATi" sent shivers down the back of geeks. Things change. I used to be a huge ATi fanboy but then I had a slew of bad experiences with them.
But either way, a 5850 is on the way. Let's see how it goes. Thanks for the advice, everyone.