Beckham has a good attitude, that should help, but I have never been a big fan of taking two guys out of their best position, especially in this case to pacify, a player, who is more than likely a short timer.
Just to clarify, I have no doubt Manny will be a better shortstop than Beckham, and I think it is a given that Beckham will not be anywhere near as good as Manny was at 3B.
The question is, will Manny at SS and Beckham at third be better than the reverse?