Before the sky falls on everyone -- the only reasonable way that the Celtics would lose Rondo to the Lakers in free agency is if A) they were willing to give him a larger contract (highly unlikely, since the C's will be able to offer him the largest deal out of any team in the NBA) and B) if Rondo thought that the money he'd be giving up by signing with the Lakers would be offset by the increased likelihood of winning a championship.
Since the Lakers suck -- arguably worse than the Celtics do -- and play in a conference where making the playoffs, let alone a deep playoff run, is significantly harder, it's a pretty safe bet that Rondo won't be bailing for Los Angeles. Unless he really, really want to play there, but there's not a lot of evidence to support that position.