Could be a number of things... I'd guess possibly faulty temperature sensors, or perhaps blocked vents/heat sinks (from dust), or even a problem with the thermal paste.
If you're fairly brave and careful, I'd suggest taking the bottom plate off and inspecting for dust build-up around the fans/heat sinks. If everything looks normal, the next safe step would be to take it to an Apple store or authorised service provider.
EDIT: If it's the MacBook Pro from your signature, it's obviously the top section (with the keyboard etc.) that comes off. Not much harder than taking the bottom off unibodies and later, just watch out for the ribbon cable that's in the middle (i.e. don't take the top off and suddenly yank it, detach the cable from the logic board first).