Update at the bottom.
I've had this annoying heat problem with my 15" MBP (it's a "Sandy Bridge" model from August 2007). If there's heavy load on the CPU (for instance when importing/editing/exporting footage in iMovie), the CPU temperature will go up to 100°C and stay there. The fans will then kick in and rev up to 6000 rpm (the maximum), but it doesn't bring the temperature down. (I assume it helps keeping it there, though, otherwise the laptop would probably catch fire)
I've already tried putting the laptop on 4 little plastic-bottle-caps to make the air-flow underneath the laptop easier, but it doesn't make any difference in terms of temperature. The MBP doesn't have fans underneath, so that was kind of expected. But I still like to use this method to keep the extremely hot bottom of the device away from the desk, it can't hurt.
This is really annoying, since a side-effect of the high temperature is that the CPU will clock itself down (thinking it will bring the temperature down). That means that, besides the high temperature (which certainly isn't good for the parts inside) and the noise, I have less CPU power than I normally would, at the time I need it most.
So yesterday I tried running an iMovie-import after taking the battery out (still using the 4 bottle-caps).
And lo and behold: Problem solved!
I could have sworn that I tried this before, but apparently I hadn't.
Without the battery in and with the CPU at full load, the temperature stays just under 60°C. At that temperature, the fans don't even go over 2500 rpm. (barely over idle-mode) If I manually turn the fans into Typhoon-mode (6000 rpm) with smcFanControl, the temperature goes down to under 50°C.
That's incredible, given that usually, with the battery in, my MBP runs at about 60°C (idle, that is).
Update: I noticed a couple of days later that performance was really bad when playing Flash-videos (YouTube HD, etc.). I did some digging and apparently all Apple Laptops automatically clock themselves down if they run on a power adapter without the battery being inserted. (my processor is a 2.2 GHz and it clocks itself down to 800 MHz, I think) There are tools out there that can circumvent this, but I really don't feel like fooling around with this. So back in goes the battery and up goes the temperature. :(