I am a developer and most of the time I do use a MacBook Pro, but I also have some experience with development on MBAir and even a Mac Mini. Here are my general thoughts:
1. The vast majority of the time you're going to be editing code, not compiling it or even testing it. Editing is a very "lightweight" process that doesn't need much CPU, so there's actually a good reason to go with a lower-power machine like a MB or a MBAir.
2. You will probably use more disk space than you think, and with more and more machines having SSDs built in (non-moving parts; faster; smaller; more expensive than traditional HDs) you'll want to have at least 500GB storage. That's fairly typical on MBPros but usually an upgrade on an Air or MB so factor that in when comparing prices.
3. You WILL spend time debugging in the emulator, and emulators are processor hogs, so an i7 will be better than an i5 for testing your code, but it comes at a cost (both money and in battery life) and really, and i5 is a really great processor. Make sure it's at least a dual-core i5; don't go for a dual core i3 or ANY Celeron processor. I also don't really think the m5 or the m7 in the new MacBook is up to the task. You'd really feel it in the emulator I think (though I've never tried).
4. If you intend on also making Android apps, know that the Android tools are MUCH slower than xCode - especially the simulators - and a really fast i5 will still seem sluggish on those (even a really good i7 seems sluggish but livable). This is also true if you intend to run Parallels or any other Windows VM software.
5. Screen size is a pretty big deal with both editing and debugging. There's a world of difference between 15" and 13" when you're trying to see both code and an emulator right beside it. You can do on-device debugging with a 13" screen but emulator debugging is really tough unless you have very, very good eyes and can shrink everything down to fit. The MBPros with Retina cost $700 more (starting at $1799) but also have 512GB SSDs and a faster i5 than the "base" model at $1099. You'll really appreciate the Retina screen while debugging, since all the devices are retina too. It's not a deal breaker to not have it though, since you can shrink the emulator windows and you can do on-device debugging to see the high-resolution versions of everything.
So the short answer is: Yes, you could do xCode development on a MBAir, but only if you're sticking with iOS (no Android/Windows) exclusively. The extra battery life on the Air might actually help you (make sure you close the emulators when you're not using them because they use a lot of battery while running). On my 15" Retina 2.6GHz i7 w/8GB I only get about 3.5hrs of xCode development time before it's time to charge (Running the emulator say, 25% of the time...). It gets pretty hot on the lap, too, plus it cost about $2,400 (w/500GB SSD). I think you'd get longer development time out of the Air.
If price is your biggest concern and you don't have to be portable, you might consider getting a Mac Mini w/i5 (the $699 version). Just know that you'd need to provide your own monitor, keyboard and mouse, but it's all the same components of a decent laptop without the cost of the screen pushing it up.
Hope this helps!
