We used this series from Steve Fabian to authenticate an external application using a JSON request (in our case it was another one of our corporate sites but the concept would be the same for an iPhone app:

Part 1 of the series:
http://www.gooddogs.com/Blog/tabid/97...

With the new services layer "baked in" to DNN the work done here in writing your own WCF services would be replaced by core services. Although there doesn't seem to be much in the way of documentation and use caes for these services yet, the heavy lifting should be there on the app site.

One recommendation/departure from Steve's article...we changed the JSON method from a get to a post in order to return the authorization token....security guys didn't like the idea of credentials being logged anywhere in the stack.

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Steven Webster
Manager, Community Platform
F5 Networks, DevCentral