So the past two clients I\'ve been at, all the talk has been about creating an iPhone app, and I won\'t lie, I want to make one. Or at least learn how to make them.
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I would recommend a laptop. It sounds like you're a contractor, so the ability to use the simulator in a demo on site may be useful to you.
In the Bay Area, it's useful to take to the frequent meetups and what not.
A low end macbook is about 999 right now.
The fortunate thing about developing for the iPhone is that you generally don't need to be concerned about getting a top of the line development machine. You are developing for a device the runs on a much more constrained hardware set. I have a MacBook that I use to run Xcode as well as Windows XP and Visual Studio 2008 with no issues. A Mac Mini should be fine, as well as just fun.