Although it was developed at Bell Labs, I doubt that the people who first built the UNIX operating system ever imagined that it would actually find its way into a phone. Granted, it's a phone with several hundred times the processing power and a thousand times the storage capacity as the original PDP-11s on which UNIX first ran - but it fits in a pocket and makes phone calls.
Daniel Eran Dilger has written a series of articles poking around inside the new iPhone, and once you get past the iWeb layout, the rightous smiting of Microsoft and their press lackeys, and the Photoshop collages, he has some interesting things to say about how the iPhone uses OS X.
I personally figured that it was just sales bluster when Steve Jobs said that the phone ran OS X - but I was wrong. Turns out it gives Apple a lot more control and flexibility with the iPhone - and they could build things more quickly in-house, since it's an adapted version of their main OS.
My next phone will probably be an iPhone. I don't get new phones very often, so this will probably be in 2011 or something like that. Until then, my beat up RAZR will do perfectly fine. I'll just tape my iPod Nano to the back and pretend.