I don't see why people are saying not to do the hack. Brad always says how it voids your warranty, but it's a simple matter of resetting your nvram (a similar procedure to performing the hack in the first place) to undo it, and at that point there's absolutely no way for them to tell you've done it. This is similar to installing a piece of software on your computer. If something unrelated to the screen breaks and you send it in for warranty, just undo the hack and boom, you're set. Hell, even if you don't undo it I don't think they'll get on your case. I say this from personal experience - I did the hack, and actually sent in my iBook for repair without undoing it, and had no problems.
Apple does not have a reason other than crippling the iBook to do this. I cannot think of a single laptop being made today (PC laptops included) that has extended desktop intentionally disabled. I'm sure there are some that won't work in clamshell mode, due to heat issues, but there is no reason to turn off spanning except to cripple the iBook. Hell, I hardly even used my iBook's newfound spanning capability but I ended up performing the hack anyway as an "F.U." to Apple.
Yeah so anyway... extended desktop ought to work great on the iBook. But you have to leave the iBook's screen on; there's no way of turning it off or closing the laptop when you only want to use the external screen. This much I can understand, I guess.