You will be suprised, but the biggest noise source is the hard drive, not the fan. the fans are usually extremely small and weak, and are located at the BACK of the laptop, shooting the noise away from you. Whatever noise is going through the case is getting blocked almost entirely.
THe hard drive, on the other hand, is much closer to your ears, and it only blocked by the keyboard and a thin piece of plastic. Moreover, the CD-ROM drive will drive you crazy; it is much more audible than a regular's PC CD-ROM drive simply because it's some 15-20" away from you.
I had to make custom noise-proofing for my laptop's HD.. dual cardboard layers, some cotton balls and special foam... still noisy, what a bitch!!! I hate laptops.