Quad SLI (as in 4 cards) is supported. There's no reason why you couldn't do two dual-GPU cards, each on 16 lanes a piece (8 lanes per GPU). However, I would STILL rather have dual GTX 580 3GB cards. That extra framebuffer will be more beneficial than simply throwing more GPUs into the mix.
With that board, it's simply Nvidia's SLI drivers that you're depending on. Unfortunately, I can't help you there.
BTW, that's a kick-ass board. I wouldn't mind sporting one of those in my current system. I also love that Gigabyte's competing board for LGA1155 (found here
) is not only $100 more expensive, but the PCIe slots on the intel board aren't spaced right. In that respect, AMD has made a strong case for AM3+.