No, in general you are unable to, though all of Apple's Intel models so far, except for the Mac Pro use PC5300 (667Mhz DDR2) RAM.
True, but the Mac Mini's use normal dimms, and the laptops use SO-Dimms (not sure about the iMacs) but don't think you can change a desktop's ram into your notebook and vice versa.
Is it possible to transfer an Ibook Ram to a Macbook?