I haven't noticed any difference in MPG with and without the A/C. I have done the same drive (to/from Vegas) on severeal different occasions and sometimes had the A/C on - other times did not. I always fill up at the same gas stations when leaving and coming back and pretty much always within a 1/4 of a tank variance, which I've generalized by how much stop and go I had to deal with.

It seems logical that A/C usage should matter, but I am yet to notice any significant difference.