The fan in my Macbook gets pretty loud when the CPU's are being hammered, which usually happens if:
I'm watching something on YouTube (normal, and dies down)
I'm viewing widgets (I think "I Love Lamp" is pretty compute bound, normal)
Screen saver is invoked (iTunes artwork)
iCal decides for some bizzare reason that it needs to launch this daemon called "SyncServe", which appears to do absolutely nothing but chew up every freaking cycle of my dual cores for no apparent reason. I have to go into Activity Monitor and kill the bugger (force quit) then my fans quiet down.
I'd look into something like the last item. Launch Activity Monitor and see if there's something hogging the CPU, thus making your fans go nuts. I'd gather that these new Intel Core 2's are powerful enough that even if you have something maxing out the cpu, you won't notice any slowdown, but you will notice the fan going non-stop.