I think many people are saying the right thing, but it's all quite confusing;
jds is correct: the difference in air pressure isn't nearly enough to affect the measurable density of any water-based solution (using a homebrew hydrometer).
jkarp seems correct but isn't explaining it enough. I'll do the math for everyone:
Bulk Modulus: "It is defined as the pressure increase needed to cause a given relative decrease in volume."(wikipedia)
Bulk Modulus (K) for water is 3.12 x 10^5 psi. The difference in air pressure between sea level and High Desert Brew's home is 760-610 torr = 150 torr = 0.20 atm = 2.90 psi.
2.90 psi / 3.12 x 10^5 psi = 9.3 x 10^-6 or ~0.001%
Assuming everything else is equal and that the density of water at sea level is 1.0000, then the density of water in High Desert Brew's house is 0.9991, a negligible difference.
Keep in mind here I am saying "density" but specific gravity can be substituted at will since we are talking about water. Along the same lines, if the water is less dense at higher altitude, then the water acting as the solvent in wort is also less dense to the same degree, so any differences would be the same for both.
I would listen to jds--I have made the same mistake of measuring the gravity of wort that I had just diluted and it was way low. I shook the fermenter around to make a more homogenous solution and took the measurement again and it was right on.
And of course I would also listen to SmugMug and RDWHAHB