I don't think he ever officially retired. He was with Denver, and they were going to let him go. They eventually did, obviously.
I just don't get this signing though. It makes as much sense as Dante Culpepper signing with the Lions.
Howard wasn't retired, but he pretty much could have been. Why come back just to play with a crap team? Surely a former all star who still has some offensive ability for 10-15 minutes a game could find a home with San Antonio, or Boston, or some other contender. I wouldn't mind having him in Cleveland if we didn't already have so many big men on the roster.