well, in the 13 MBP, it seems the graphics performance of the Sandy Bridge CPU + IGP is on par, sometimes better, than the C2D + 320M, so, in this case, they didn't really sacrifice graphics power.
the 13 MBP uses a more powerful CPU than the MBA would, so, the weakness of the IGP, which can be carried by the Pro's CPU, would be expected to show more in the Air. this, as i understand it, is the problem.
for example, see the 13" Samsung Series 9 for a worst case scenario. In fact, this seems to be a good model for the potential 11" Sandy Bridge Air.