How to authenticate to a webservice via iOS?


I finally finished the book and I must say it was amazing.

The webservice chapter was very well done however one big question remains, how do you authenticate the user?

I am working on an application that could be used via web or iOS. Therefore I would be holding the data in Google app engine. Now when the user tries to make a call to my webservice, I need to authenticate the user to only return the items that belong to him.

In web world you have sessions where a user authenticate and the session is set. Surely we don’t have this luxury within an iOS app, do we?
So how do we authenticate via iOS?

Is there any blog/resource you know that is digging more into this topic?

Many Thanks