Taken straight from the US Airways site:
“Join now and you could get 25,000 miles.
Now’s the time to join Dividend Miles. Every time you fly in your first 90 days, you’re one step closer to award travel. All you have to do is join.
Fly at least one segment and you’ll get an extra 500 miles in your Dividend Miles account. If you fly 8, we’ll fast forward you to 25,000 miles, and that equals an award ticket.
Take 2 roundtrip flights, each with a connection, and you’ll have 8 segments – it’s that easy.
* Join Dividend Miles between October 19, 2009 and January 15, 2010.
* Fly between 1 and 7 segments during your first 90 days and you’ll get 500 bonus miles.
* Fly 8 or more segments during your first 90 days and we’ll ‘round-up your miles’ to 25,000 miles, enough for a domestic roundtrip award ticket.”
Full details can be found here