Funny, I just tried this yesterday with my 5850s. I've been running at 0.95 and took them down to 0.90. In my case, at least, the reduction in power consumption wasn't very significant. Sorry, I don't remember the exact difference, but I think it was less than 15w for the four 5850s in my rig. Anyway, I decided not to bother with it and stay at 0.95. But the cards were stable at 0.90 during the time I played around with it, and I took them up to 725MHz, just 25MHz below the core clock I normally use at 0.95. I use atitweak (linux) to set the voltage if you are wondering.
Since I see myself facing a situation where I'd prefer to "run my gpu's to the ground" I find myself looking for the utter optimal MHS/W, in advance for the upcoming block-reward drop. It can also be the more profitable way to go for me, 5970 2nd hand markets seem to be getting active around here, most likeley because of new models hitting the market and I fear the situation might get rediculous around the block-reward drop. Personally if I had free electricity I'd make damn sure to have liquidity to invest in to gpu's when the block reward gets chopped down.
I agree - think this is a good strategy, and for those with access to cheap electric it makes more sense than going the FPGA route, at least for a modest size operation. You might also consider 5850s if you can get them cheaper than 5870s. I'm getting 300 Mh/s out of mine at 0.95. I think its worth trying below 0.95 to see if the power saving is worth it for you. Just saying that for my modest rig it didn't really seem worth it. While I want an efficient rig, I also want my rig to run more or less like an appliance that chugs away rock-solid stable without any fuss, and I'm not that motivated to futz around with lower voltages if it's only going to make of difference of 15-20w or so, especially if this means a reduction in hashing rate. Power consumption depends exponentially on voltage, but an exponential curve has a rather flat bit before it ramps up, and I think that when you go below 0.95, you are starting to get into the flat bit where lower voltage doesn't get you as much power savings. This at least is consistent with what I've observed for my rig in playing around with different voltage levels.
Edit: After playing around a bit more, I'm now skeptical that atitweak is actually lowering the voltage below 0.95. At the same core clock, I see no difference at all in terms of what I'm pulling from the wall when I use atitweak to supposedly drop the voltage from 0.95 to 0.90. The difference I saw before was I think entirely due to a slightly lower clock. Sorry about the confusion - I shouldn't have taken what atitweak was reporting at face value. If anyone knows how to drop 58xx voltages below 0.95 in linux, I'd like to hear about it.
