Core 2 Extremes won't be limited to desktops only fairly soon, with Intel planning to release the Core 2 Extreme X7800 for the notebook starting in the third quarter of this year. With a stock clock of 2.6GHz, 4MB of L2 cache and of course multiple cores, it will be an absolute behemoth of a chip for a notebook. It'll be succeeded by an even faster model, the X7900, later in the year.
Probably not that light on battery life, at least under load, these chips are definitely suited to an environment where you might be using a laptop as a replacement desktop. They aren't going to start out cheap, but that's expected.