Shield always stacked with mage armor, as it provided cover (pre-errata) or a cover bonus to AC (post-errata/FAQ). Interstingly, this meant that shield didn't stack with hiding behind the fighter or in an arrow slit or the like.
Now, shield stacks with hiding behind the fighter, but doesn't stack with an actual shield (or a ring of force shield).
I wonder if a ring of force shield will have any new abilities (as an essentially un-enhancable free action to store, no encumbrance to carry shield, I don't think it was really worth the 8.5k it cost).