user.login not updating sessions table

I'm trying to log in using user.login from an iphone, using the following process:

1) open a session with system.connect
2) get sessid from response
3) Use sessid with username and password to login the user using user.login
4) Get back new sessid and user data

This works. In step #4, the user data is properly returned. However, when I look in the database at the sessions table, I can see that the UID for the sessid in step four is 0.

I have gone one step further and used:
5) access services with the uid and the sessid (from step 4), using user.get

In this step, the user data is again returned... but in the sessions table in the database, the user is still user 0.

I can't figure out what's going on. It seems to be logging me in, as it's sending me the user data, but as far as the system is concerned, I am not logged in, as the sessions table is never updated.

I'm using a clean install with the services module, the services_user module, and the JSON-RPC (server) module. I don't have any other modules installed, and I'm really not sure what to do from here.

Thank you.