Entirely different idea for the ending
http://www.memoirsofa.com/FAQ.html#SGROBJ7D89116023151B40
Partway down this page is a short interview with Gabrielle Zevin, the film's writer; in discussing the ambiguity of the ending, she says that it could be inferred that the two are actually in the cab together, have been married for the film's duration, and have been playing an elaborate game with each other all night. The fact that this idea comes from the writer is particularly jarring; I completely missed this possible angle to the movie.