What's the highest rating any computer has ever attained? 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.