Fully stock, 4.0 5spd manual driving about 90% highway gets me about 24mpg consistently. Nothing that special I do, I just drive long distances all the time and that happens to be highway. 24mpg may be good, but not when you're driving 400-500 miles a week...

As if to verify what I've been saying about keeping the RPMs down and keeping it under 65-70 on the highway, I'm doing the opposite lately and my mileage has plumited! I am commuting w/ a co-worker now, so I haven't fueled truckzilla up much lately. I decided to try driving 'normally' and more agressively. Sure enough, my last tank-which was mixed between driving agressively and not-netted a relatively poor 18 MPG. ... But then it was a helluva lot more fun leaving the toll both at 4k RPM!
I've heard the opposite; that altitude & the thinner air... I've been reading a lot of stuff about Miatas lately (I desperately want a toy car). I recently read on one forum about a guy out in Colorado who stripped his car down, advanced the timing, and milked an unheard-of 45 MPG out of his car. He attributed a lot of it to the altitude he was running at.