I would never trade Bynum for Josh Smith. Sure, he's a good player, but won't have the impact of Bynum. Bynum can alter shots just with his presence - No need to even go for the block every time. Mitch has made it clear that he's not making ANY changes to the Lakers team this year (except for maybe Sasha/Luke, if someone would take him).
Also, Bynum would easily be playing now if his doctor chose to just clean up the meniscus (which is what Branden Roy had). He elected to repair it at the time of surgery, which has a longer recovery time, BUT is much better for the future, hence limiting his future injuries (on that particular knee, at least).