I came across this on digg this morning. This guy took the 802.11n card out of a Mac Pro and swapped it with his Macbook 802.11g card. From what he says, it's a pretty straight forward swap. The three antenna wires, in the C2D Macbooks, are not an issue. The Mac Pro card only requires 2 antenna wires. Here's the link:
Macbook 802.11n upgrade
Macbook 802.11n upgrade