thats strange... is your ps3 in a hot location such as a cabinet with no air flow? Or a room thats not normal room temp?
The ps3 doesnt run hot normally so it can be a number of things. One is to check to make sure it has airflow and that none of the vents are blocked . So depending on how you are laying it down, it might even be a good idea to put some thing under it to let air flow through (kind of like when a laptop gets hot on the bottom, you put it on top of two books leaving the center of the laptop not touching anything). If you are in a hot location where the heat is part of where you live, a regular fan would help a little and an airconditioner would be key.
Also DUST will cause anything to overheat so make sure its cleaned out (can of air fixes that).