I want to buy a MacBook to develop iOS apps. What are the minimum requirements?
Today, I'd go with either a 15" or 17" MacBook Pro. They would allow you to work with the iPad simulator at its native resolution. It's a bit too big to run full-size on the 13" models, so I always end up running it at half size, which is less than ideal. Even if you're not planning to develop for the iPad right now, I'd still plan the hardware purchase around that possibility. The likelihood that you'll do iOS development and never build for the iPad is pretty low.
Beyond the simulator issues, I like to have the largest screen possible when I'm programming. I have a 13" MacBook Pro, but I connect it to a 24" monitor when I'm at my desk. If you're like me, the 17" would be ideal, if you don't want to have a second screen.
However, Apple is holding a Mac-focused special event tomorrow, and it's never a good idea to buy a Mac this close to one of those events. The entire laptop line is due for updates, and there has been (unconfirmed) speculation that any new models may run at higher screen resolutions, especially on the smaller end of the range.
MacBook Pro 13" with an external display (22 or 24 inches), keyboard and mouse.
Later you can upgrade it with 8 GB of memory and change its disk to a SSD or Hybrid disk if you need more performances.