The Cell is not that great a processor for general purpose desktops. The core processor is a stripped down PPC that, from what I've read, would kinda suck at the random branchy code you would expect in a desktop OS. The "cells" are just independent vector processing units, useful in certain situations but you aren't going to be doing things like running OS processes on each one.

Even if IBM could combine a full G5 with the cells, they couldn't get the G5 alone into suitable form for a laptop. Adding 8 cells to that would make the job even harder.