[aredington]

The ring is still ambiguous. It is never characterized properly as a totem (Cobb never uses it awake, that we see). If you interpret it as a symbolic representation of his mental affiliation with Mal, then:

a) At the end of the movie he is awake, and he really meets his children. The lack of the ring is consistent with all available evidence prior to this point because he only has the ring on when he is asleep.

b) At the end of the movie he has successfully escaped Limbo twice. Cobb has mastered his own psyche; he has achieved closure on Mal's death; he has finally embraced in his own mind that the Mal in the dream is just a ghost, and can be in the dream without the ring. He can dream again without Mal invading from his subconscious.