It kind of sounds like this is a 'maybe future hobby' versus much real dev experience?
I can tell you that screen real estate matters to me as a software engineer, especially when looking at code day in, day out. RAM and a reasonably fast CPU are also worthwhile, but it depends on the type of development anticipated. iPhone apps are smaller, not exactly multiple GB of source code to be compiling like for a big enterprise Java app..php or other web development in general also tends to be smaller in scope, which means CPU and RAM are less of an issue, although if you plan to also run a web app server on your laptop that can offset the CPU and RAM 'savings' of writing smaller apps.
If your primary operating mode will be with an external display, then the resolution doesn't matter as much, either.
I've got some projects that take multiple hours to build, some 15-30 mins, and some 5 mins or less, and/or are interpreted/can apply and test changes near instantly, but need a DB or other servers running. I also need to be running at least two VMs at the same time, which both RAM and CPU come in handy for.
There really is no 'best for development' it's what fits your wants as well as meets actual needs (if any are indeed needs). For me, a 1440x900 res or bigger, 8GB of RAM or more, and 500GB or more of storage is the best compromise laptop/near desktop replacement for ME. Technically, a Mac Pro might be better, in that I could kit one out to 16GB or more much cheaper to be able to run more VMs, but then I'd need to ensure work across multiple machines was always and correctly kept in sync, so my personal choice was a 15" MBP as a decent compromise all around, and to not worry when I need to run into a meeting with laptop in hand, if all work is synced back and forth or not.
If you're buying this 'for personal use,' and maybe a 'hobby' to learn iphone app development or similar - buy what appeals to you personally, and once you understand your *own* real needs, you won't need to ask the question at all.

I wouldn't expect any iPhone development to 'require' anything a MBA can't provide, the rest is really just comfort and speed.