Here's a curious thing. Gearlog is reporting that in testing a MacBook Pro, they found substantially lower performance while running on AC power alone, with the battery removed. Apparently, Apple doesn't hide this quirk; there's a Support Document claiming that this is done to prevent "the computer from shutting down if it demands more power than the A/C adapter alone can provide."
On their new unibody MacBook Pro, Gearlog noted a 36% lower processor benchmark without the battery. The upshot is simple: leave the battery in, even if you use your MacBook (Pro) at your desk. Interestingly, at the same time Apple does recommend allowing the battery to discharge at least once per month.