The Chiefs haven't announced (or confirmed) the news.
As a Chiefs fan who suffered through the 2009 season, this is a very cool and very surreal moment. The dude many of saw struggle in 2009 turned around and put up, frankly, one of the best seasons in franchise history.
If you were wondering, this is one of those stop and smell the roses moments. We can scrutinize Cassel all we want but at the end of the day his numbers say he was one of the top quarterbacks in the AFC.
Presumably we'll be writing another post on this when the Chiefs confirm the news.
From my view, Chiefs fans really started to embrace Cassel after the Seahawks game (with that image of him running down field throwing a block for Jamaal Charles) and that continued when he missed just one game because of an appendectomy.
How cool is this?