I'm guessing the weight of the PS3 suggests a design that utilizes "thermal mass" to a greater degree than typical designs. I'm sure it has an aggressive active cooling system, as well (the usual fans and extensive heatsinking). The trick with the thermal mass is that you don't need to dissipate heat so frantically as it is generated. You've got room to store the heat so it can be dissipated at a more steady rate, regardless of heat input or processing load.
Ultimately, the console will get hot under full load, but the time span for this event is well decoupled from the actual activity of playing specific levels in a game. Say, after 2-3 hrs of solid, hard playing, you may notice the fans starting to do some heavy breathing for the console. Naturally, the racket is unavoidable once the fans start running hard. They will also keep running for a long while after you let off on the game playing (maybe 15-20 min?). It cools over the longterm, and probably some intelligent/dynamic fan speed control based on the profile of existing loads (sporadic, continuous, non-use, moderate loads).
Potentially, there is liquid transfer involved to more effectively couple the processor cores to a greater portion of the heatsink/heatmass structure. The heat "sees" more available heatsink than in the typical configuration of a heatsink slapped right onto the core.
