I'm building an app for smart phones that read/writes to extra tables on my DNN site.  To perform the read/writes the app user must be validated on startup and pass in their userId with every request.
I'm thinking to achieve this the user must first create an account on the DNN site.  Then to use the app they would enter their user/pass in the app that would be passed up to a WCF service to be validated against the DNN database which would return their UserId from the DNN database.  Their UserID would then be held in the app until the LogOut deleted it.

How would one approach this?

Hi Sebastian.   Great to see you still here advising people :)

I was wondering if I could follow the path of the DNNlogin controls?    I would need to know how DNN does it online when a user is signing in to the website.  Obviously the user/pass is put in for validation.    What I'd like for my app to webservice is to pass in the user/pass and return the userIdGuid.  I could then use the GUID for every transaction following from that device to the web service.
I suppose what I'm looking for is the functions that DNN is using for validation.   
Any more ideas? 

Thank you Sebastian.
The answer as always is quite simple once you know how.   Everything is around the System.Web.Security / Membership class.   The best place for anyone to start is this video by Joe Stagner...
http://media.ch9.ms/ch9/f97e/59fc3d40...

With only a tiny bit of code I have it working in a basic level where I can put the username / password into a mobile app and the DNN site UserID Guid is returned.  It is important to note Joe Stagners security warnings about "packet sniffers" etc grabbing the information.