Well, did it say how long he was gone for? I don't remember. They didn't go too much into the specifics of the background. There was no real explanation of how long the dreaming technology had been around, and it was apparently fairly common. The explanation of events outside the main story were vague at best--including the details of Cobb's flight from the United States. So, if he had been gone for *years* then it would make sense, but if he was only gone for a year or less it wouldn't be too bad.
Inception includes enough mysteriousness to make you question where reality ends and dreams begin, or if there was ever any reality in the movie at all. Personally I like believing the concrete interpretation as it makes for a nice, happy ending.
