US Airways is offering 5,000 bonus miles after every three Shuttle segments through May 31, 2013. This includes their nonstop service between Boston (BOS), New York (LGA), and Washington (DCA). You can earn a maximum of 50,000 bonus miles through this promotion. Registration is required using promotion code SH5K, and bonus miles should post one to two weeks after travel is completed. It’s worth noting that even itineraries involving other segments are eligible, so if you’re flying from Tampa to Boston via Washington DCA in both directions, the two segments between Washington DCA and Boston would be eligible.
(Tip of the hat to Gary)