The idea isn't a new one, but I think it is worth talking about. If the Nets knew they couldn't get Howard and didn't have a chance of keeping Williams, then the Celtics would be a very logical team to contact I think.
Here's an idea: Deron Williams for Rajon Rondo
One person in the Nets organization who is not directly connected to negotiations with Williams offered his opinion on the star's intentions, saying he doesn't think there's any chance the guard stays with New Jersey. He also thought the team would be wise to come to grips with that and trade him before the March 15 deadline. If Williams is available, and you're the Celtics, don't you dust off the Chris Paul trade folder and go hard for him? You'd have to part with Rajon Rondo and someone else (Jermaine O'Neal fits neatly from a cap standpoint) to get the deal done. But it's worth it if you're the Celtics.
Of course you'd need to get some assurance from Williams that he'd stick around, but given Howard's preference to play with Williams, he might be enough to entice Dwight to Beantown. I think it would at least be worth a shot.