It's not the slam-dunk the author thinks it is. In addition to the evidence laid out by previous posters, there's another clue that allows me to maintain that the ending is truly ambiguous. The evidence starts with the scene where Ariadne intrudes into Cobb's personal dreams and forces Cobb to it that he needs these sessions because "... in my dreams we're still together". Since when Cobb is dreaming he and Mal are still together, he continues to wear his wedding ring in his dreams. BUT in the climactic scene in limbo near the end of the movie, he faces the reality that her presence there is "just a shade". He lets her go. Now, even in his dreams Mal is dead, and he need not wear his ring.