This guy is about as well-trusted as it gets when it comes to internet rumor sites:

http://www.draftexpress.com/viewarticle.php?a=1369

To put it simply:

Phoenix trades: Shawn Marion (doesn't get along with Amare, max contract), #27, possibly Barbosa

Seattle Trades: Rashard Lewis (Can opt out after next year), #10, possibly Fortson

Basically Phoenix does it to stay competitive for now while extending guys like Diaw, knowing that Lewis is likely a one year player.