I plan to develop an iPhone app with my friend. Each of us is specialized in different parts of the app. For example, I am responsible for interface design and server-side programming while my friend for image processing and computer vision. We would like to share the income of selling the apps, but have no idea how exactly we can share the income. I guess that App Store does not allow us to split the income automatically into separate accounts, right? What do people usually do in this situation? Just simple bank transfer?
A simple bank transfer will not do, as you have to justify to the IRS why you are giving money from someone else. But you could worry about it later, once the amounts are significant enough. If it's less than a few hundred dollars, you probably fall into some kind of exemption.
You could also create a limited liability company (with an operating agreement in place about how you split your revenue - you should write one anyways), get a merchant/business account for the LLC (through a bank or other provider) through which you receive the money, and split the transfer from the merchant account to your personal bank accounts. Note that there are formation fees and taxes involved with the LLC.
The important thing is to put an agreement in writing, setting out your agreement on how the app is owned and how income is to be shared.
That keeps things clear between you and, if things go well, gives you the basis for putting the app into a business or selling it on to a third party.