>What is the purpose of this, other than just to see if it can be done? I can't think of any software that would be worthwhile emulating that doesn't already have a Linux or Windows version (which is covered by Wine), other than xcode.
It's not about having a "Linux or Windows version", it's about how good OS X's version is. For some people, there are NO alternatives.
I would suggest to these people: get a Mac. If you're seriously fretting over having no alternatives, then you should be getting the Real Thing. Is it really worth trying to "stick to the man" or waste time trying to deal with all the inevitable bugs when running on a non-native platform?
If you seriously want to spend a couple hundred bucks for a netbook to run Linux and this OS X emulation layer, you're not going to get any serious work done for A) slowness and B) bugs. You'll have a cool toy, you can stick your tongue out at Apple, but you're not going to be designing, developing and compiling that Next Big Multimillion Dollar iOS Game.
Hell, even Mr. Linux himself (you know, that Torvalds dude) uses a MacBook Air. If he wants to run OS X, he can just dual boot.