Via One Mile at a Time channeling TMTravelWorld, US Airways is offering 500 miles to new members who sign up and fly one segment in the first 90 days of membership and they’ll round up to a full 25,000 miles if you fly 8 segments in 90 days.
If you’re going to short-hop around the Northeast or Southeast, it could be worthwhile ot credit to Southwest. Hard to imagine a reason to construct a mileage run just for this bonus (though for the right route and dates, 8 segments can be had pretty inexpensively in a single roundtrip). But for those flying anyway and perhaps otherwise traveling Delta without status this could be a winner.