My best guesses would be that the vents have become clogged with dust, and is no longer able to expel heat efficiently enough to cool off the CPU, or the thermal paste between the heat sink and CPU or GPU is no longer effectively transferring heat and may need to be reapplied. If you feel comfortable with opening your MBP take a look at ifixit.com
. They have excellent detailed walk-throughs on MBP tear downs, and a great tutorial for applying thermal paste so that it is very effective. If you do decide to re-apply thermal paste, i recommend "Artic Silver", I've used it on my MBP, and most recently on my PS3.