Aren't the Lakers in pretty much the same situation as the Celtics? I remember this year hearing Kobe say that they didn't have the cap space to do something big this summer. I think this was in response to a question about them getting one of the top free agents.
Yes, they're in about the same situation, in that they don't have any salary cap space.
However, like the Celtics, they could trade a current player (or one of their own free agents in a sign-and-trade) to another team for a free agent, so long as the free agent decides to go there.
In other words, if Bosh wanted to sign with the Lakers, Toronto and LA would have to work out a trade that fits under the CBA trade rules. Bynum would be a good bet to go in such a trade. Similarly, if Bosh wanted to sign with the Celtics, the Celts and Toronto would have to work out a trade, which would likely include Rondo and some other assets.