In Can a baseball be hit farther at altitude (I hope I entered that link right!) I’m afraid the Straight Dope staff has led Cecil astray. (Alas, I have trouble making physics humourous. Only my attempts at understanding it are humurous.)
The Earth’s gravity at Miami is not greater than that at Denver because Miami is closer to the equator. Yeah, the surface of the earth, or more accurately, sea level, bulges near the equator. But that is because the force of gravity “bulges” there also. The reason sea level is an important reference altitude is because the force of gravity is a constant along the sea. If gravity were stronger at Miami’s latitude, the ocean would flow towards that latitude until the ocean surface there was under the same gravitational pull as the surface at Denver’s latitude. (In fact, it already has, hence the bulge.) The ocean surface is a surface of equipotential, with respect to gravity, just like a metal is an equipotential with respect to electricity. All that stuff about “centrifugal force” etc, shuld likewise be ignored.
Of greater import is the composition of the earth under Denver (or Miami). I believe, that mountains tend to lie over less dense material, which is one reason mountains form, so the Earth’s gravitational pull at Denver might well be weaker than at Miami. But you won’t calculate it based on Denver’s altitude.
I’ll barely mention that gravity gets weaker like the distance squared, so a mere 1 mile of altitude would certainly negligible for baseball.