NOTE: Question marked answered because it was forced after being put up for bounty. Actual solution was to have Apple repair it.
Trackpad usually works, but sometimes stops responding for around 5 seconds and then suddenly starts working again. Seems to happen when I switch between typing and moving + button-clicking and also when I do a lot of double-clicking. Tried turning off the "Ignore accidental trackpad input". Apple replaced the keyboard/mouse under warranty. Problem remains. Any ideas?
Edit: White non-unibody Late-2008 13" MacBook - fully up-to-date OS. Doesn't seem to matter if it is plugged in or not.
Edit: Updated to Snow Leopard - seems to have made it worse.
Edit: Applying even a little pressure to the left palm rest creates this condition. Apple replaced the top case again and this time it seems to have fixed it. Although, it looks like they may have added a spacer on the left palm rest to "fix" it.