Vidilab are reporting that a BIOS issue in one of Gainwards 6600GT models prevents it from operating in SLI mode.
It would be interesting to see if anybody else can verify this. Vidilab's article on the matter can be found here.However, in one of our tests we experienced an unpleasant and unexpected suprise. The very top of 6600GT cards, a card with a very short name (Gainward Ultra/1960 XP Golden Sample "Goes Like Hell", works at 540/1150 MHz), refused to work in SLI mode.
Meanwhile, Leadtek's and nVIDIA reference 6600GTs work just fine, so you can't blaim the motherboard (which had its share of problems with drivers, BIOSes etc.).