Hacker Eddie Zarick has done just that—taking a 4.3-inch LCD and mounting it in the place of the existing logo light.
Zarick's also done us the pleasure of telling us mere mortals how he pulled off this snazzy trick. He writes that the hack requires a combination of the following items: a white or black old-school MacBook (he's not sure if the new unibody machines will work), a Century Japan 4.3 USB LCD screen (currently selling on eBay for about $200), DisplayLink's USB LCD drivers, a clear Apple logo (about $10), soldering iron and solder, "misc tools"—and heck, a lot of guts.
The mod involves taking your Mac apart and soldering some wiring to your logic board. However, Zarick writes, you'll essentially lose the use of one of the notebook's USB ports: "Whenever you plug something into it it'll power off the back screen and not power your device. (at least that’s what’s happened to me). If you can find another source of 5v power on the board that should do fine! But I'm not that good."
If that whole process is too scary for you, you can buy the already modded MacBook direct from Zarick on eBay. As of this writing, the price was about $1500.
My question is this: does the coolness of the mod outweigh losing however much battery power that this extra screen will require? You be the judge.