Along with that, I'd like to know what the highest rating of any human has ever been - including historical estimates.
This page has a lot of good info. Kasparov is the highest at 2851. However, such comparisons are fraught with peril as ratings inflate, and they really can't be compared from era to era. I don't think it will be long before Magnus Carlsen takes the 'highest rating' crown. This won't mean he's stronger than Kasparov at his peak.
Computer ratings are trickier. Rybka 3 claims over 3200 Elo. If this is the true, then only about 6 GMs in the world have a statistical chance of defeating it.
EDIT - that 3200 rating is on specialized hardware. I think on a common (but beefy) PC, the rating is about 100 pts lower. This means that currently there are about 30 players in the world who have a statistical chance of not losing. None of them have an appreciable chance of winning. Don't be depressed - no human has an appreciable chance of beating a car at a race. Somehow this didn't make track and field less interesting.
EDIT in 2022: Stockfish is now at 3546
According to this source, Computer Chess Champion Komodo has a rating of 3344 in 40/40.