Currently, US Airways uses 727s and 320s on the old bos-lga-dca route. They use 733s on the bos-iad and bos-dca routes, and DC9 on lga-iad. I've heard also that the shuttle will be all 320 eventually, but I doubt that. A 320 is bigger than a 733, and a whole lot bigger than a D93. I would be less surprised to see 319s replacing the aircraft on the new shuttle routes.
Or, here's an idea... keep 320s and 319s in shuttle colors, and fit the aircraft to the needs at the current time. I mean, if only 100 people show up for a flight, send a 319, and if 170 people show up, send a 320. Same crew for either one, same maintenance. Hell, they could even order 2 or so 321s in case the motherlode shows up sometimes. Bam. Instant guaranteed seating to put a hole in Delta's plans. I wonder about the feasibility of this. Any ideas?