i have a 15" powerbook and i seem to be doing more tracks on the road with this and a pair of headphones than i do in the studio these days. hmm i really want to waste money on a macbook pro but at the end of the day it'll be just that - a waste hehehe.
no seriously, a portable mac is an incredible tool to have. as well as your mobile studio it also becomes your mobile office, because OSX works so well if you're on the move.
i think it's important to have a powerful desktop in the studio, cause it make you think you have a 'real' studio and of course the way you think about the studio is integrale to the way you make music. but if i had to make a choice between a desk and a lap, it would be lap every time.
ps make sure you up that memory to the highest it can go!!!!