Um, my Dell D600 fan can get pretty dang loud too. Usually when I'm doing something like running a webcam that fan will spin up and well, not exactly howl, but it certainly gets cranked up.
My Macbook can get pretty loud at times. However the vast majority of the time it's about as silent as any computer. Same with my Mac Mini. Things that will get either of them spinning up include:
*) YouTube for some reason
*) Photoshop CS3
*) iCal insists on launching this thing called SyncServer, which will monopolize the CPU(s) and get it spinning
*) Parallels
*) Ad laden sites (newspapers are the biggest offenders, probably because of flash)
*) This program which I briefly installed to see if I could use my Macbook to access my employer's VPN (CheckPoint SecurClient)
*) Rendering video (obviously)
*) The iTunes album art screen saver
*) Dashboard (probably the weather radar widget)
All of these, with the exception of that VPN software which I promptly uninstalled and abandoned the idea of using my Mac on the corporate network, are transient and the fan will quickly quiet down. There was one time my Mac Mini had it's fan going full blast for a long time but I didn't notice it for a couple of days. It's connected to our HDTV as a Media Server and I don't really pay too much attention to it unless I'm going to use it to play a movie, so when I finally realized that it was doing that it was because software update had launched and was waiting for me to acknowledge it, and apparently that was somehow CPU intensive.
Anyway, bottom line is that laptop components get hot, that's why they have fans, and sometimes they get a bit loud. Still not as bad as the computer I built for my wife pre-Mac that had both chassis and CPU fan going like a jet engine 24/7.