Most apps these days have some kind of tie-in to a social network of sorts. Which means your iOS code has to potentially authenticate a user to many different services.
The good news is there is a standard for this called OAuth. The bad news is it is one of the most error prone developer tasks I've seen recently.
OAuth is not hard conceptually like Quantum Mechanics, rather it's hard like there are a thousand little things you can do wrong that cause it not to work, and you end up spending time tracking down what little parameter is wrong by 1 character.
Normally in this situation a quick Google search will reveal lots of carefully refined libraries so you can just drop them in and go.
One problem with OAuth in finding a helper library is the major players like Facebook and LinkedIn are not implementing the standard in the same way. For example Facebook uses OAuth 1.0 while LinkedIn uses 1.0a. Just enough difference to require every library user to waste time finding the same subtle differences.
For iOS, I've yet to find a single OAuth library that supports all major services. I mean out of the box without digging and tweaking for each service. Yes I've seen things like the OAuthConsumer project and others, but again none are plug and play for the major services.
If anyone else finds one drop me a line, otherwise I'll consider releasing my own. It's too late for me to not waste time on this, but others can be saved.
We need an open, canonical, objective-c library that makes these services codable by my 9 year old son.