Btw, for those who are unaware, the ati proprietary drivers (fglrx) already support muxless hybrid intel/ati graphics - which I think is the equivalent of Optimus. It does so by interacting with the Intel's X.org userspace driver - so it feels a little laggy. So while I don't see dma-buf as a necessary feature for hybrid graphics, I hope it does makes things faster.
Funny part is with these muxless systems, you simultaneously get both KMS with the Intel driver and the good features of fglrx (nice OpenGL performance and also OpenCL). KMS otherwise wouldn't be possible with fglrx (or other closed drivers). So hybrid graphics FTW!