I won't dwell on the fact that laptops are ergonomically evil and will eventually send you to the doctor with one form of RSI or another. If you don't already understand this, it will be very hard to convince you.
Instead I'll point out that your productivity is proportional to how much monitor you've got. Researchers have measured this [1][2], hackers [3] and techies [4] have testified to this, and Apple has sold a lot of incredibly expensive 30-inch displays because of this. Why are you killing your own productivity in order to save the cost of running an electric space heater? Is it the coffee? Don't get hooked on the coffee!
[1] http://www.useit.com/alertbox/screen_resolution.html
[2] http://research.microsoft.com/displayArticle.aspx?id=433
[3] http://jeremy.zawodny.com/blog/archives/005541.html
[4] http://chris.pirillo.com/2007/05/31/dual-monitor-display-inc...