My Macbook Pro 15" (pre-unibody) was clogged up and it was time
Every 6-9 months I need to take apart this laptop to clear the fan vents of dust buildup which make the thing overheat. It's been 20mn now and through the same level of usage, my computer is running quite a few degrees cooler than it would previously : from ~190-195 back down to ~138-155 and the fans aren't duty-cycled out anymore.
They would run full-speed (5100rpm) but now they're settled at 4000rpm tops
it's not tough to take apart
running OS-X Mavericks