How can a server authenticate an iPhone app (the code, not the user)?
authentication, iphone
Solution
All other answers give legitimate ways to provide some additional, but not perfect, security. You should know up front (since no one else has been so explicit) that it is not possible to provide theoretically secure communications such that your server can always validate that the client on the other end is a purchased copy of your application running on sanctioned hardware. You can't do this because whatever hardware level security Apple has built in to kick off a chain of trust (so that this may actually be possible for them), they don't expose to you.
Your strategy thus should be one of "many barriers", some larger, some smaller, designed to thwart varying degrees of complexity of attack and sophistication of attacker. See other answers to this question for some good ideas. How many barriers you need, and of what sophistication, depends entirely on the cost to you (economically, trust, whatever) of having an attack succeed.
Do consider also the idea that if you can avoid a "reproducible" security attack, you're better off. In other words, if someone breaks your app/protocol, and the data for other people to do that is the same for every copy/instance, then you're in more trouble, because the instructions/keys can be just posted to the web somewhere. If you can figure out a way to make every copy/client unique, then you can observe on the server and at worst, cut off known broken clients, etc. (This is hard on the iPhone platform.)
Problem
Let's say I have a solution involving an iPhone app that generates some information and then sends that information to a web service for processing. It is important that ONLY requests from instances of this particular iPhone app are allowed to be processed (there may be many instances of the app used by many different users, but I want to be sure they are all using code that I trust). In other words I want to be sure that my iPhone app cannot be (easily) impersonated by other clients. How would you do this?