I recently went through a similar process but with slightly different requirements. I have ended up with a Centrino based laptop with the Fujitsu MHT2060AT hard drive. I don't know if this has more platters than the 40GB version but it is very rare that I hear it. I do feel the heat from it though as it is mounted under the palm rest. I hate to think what a 7200rpm disk would be like. When the Pentium M processor drops to 600MHz on battery power, it runs completely fanless.
I also now have a (nearly) silent server
based on a fanless mini-itx EPIA motherboard. This is powered by a fanless power supply and contains a Toshiba MK8025GAS hard disk. Again, I think this contains two platters whereas the MK4025GAS contains one (so probably no noise reason for going for the MK2025GAS) The disk is mounted within a foam frame (although this is probably not necessary). The server sits on my desk, less than 1 metre from where I sit. The only time I can hear it, is in the middle of the night when I sit still for a couple of minutes and really concentrate on the server.
So in summary, I run both Fujitsu and Toshiba disks and both are very very quiet. I have no complaints about either and for the first time in ~3 years, I have no desire to buy or quieten anything else.[/url]