I am surprised to see from this site that 1963 bills are still in circulation. I would not be surprised if paper money went out of style by 2043. We are already in an arms race against counterfeiters, and molecular nanotechnology promises to end that race with the counterfeiters in the winners circle. My best guess for mid-21st century money would be gold for small purchases (not yet worthless like that _Twilight Zone_ episode, but like today's silver), and computer database exchange with ID verification based on DNA sequences, retinal patterns and fingerprints for large purchases.
It would be strange if advancing technology made our hobby move from "fantastically futuristic" to "archaically obsolete" in one lifetime! Any thoughts?