Boy I sure wish you could do this. As it stands you're limited to memory, hard disk and maybe CPU upgrades. It's a closed architecture for the most part and the Gateways, Dells, WinBooks of the world would like to keep it that way.
<< Most laptop parts are proprietary, require precision engineering and perfect interaction with other hardware in the system. Generic parts do not offer this. >>
I don't see why that can't change. In fact it should have happened by now but laptop sales are lucrative...and, of course, there hasn't been much demand for home-brew lappies.