As the soon to be proud owner of a macbook, i've dived a little on the net about it a,d found out that while Apple advertise and tell C2D Macbooks only support 2 GB of RAM, in fact you can use 3 GB in 2+1 or 2+2 configs. (Note that the 2+2 config is still 3 GB limited but you use the 128 bit interleaved mode)

Mind that It's not supported by apple but it sure helps to squeeze every bit of power from your macbook, and helps being a good boy with the battery.

Benches on OWC/macsales
More benches at OWC/macsales

Maybe one day Apple will update the EFI of the C2D macbooks to fully support the 4GB limit of the Intel chipset… One day… sigh…