It is an interesting thought. At the start of 2012, Rondo would be 26 years old and Howard 27. That would give them a solid six year window. However, it is my contention, given the severe offensive deficiencies of the aforementioned players, that the Celtics would need a premiere wing scorer. Rondo and Howard would not be a viable championship contender if surrounded only by role players: a dominant wing scorer would be needed.
Also, it would not surprise me if Ray and Garnett would be interested in re-signing on the cheap if the C's could land Howard. It would be a one last go around sort of thing.
I think Rondo/Howard pairing would make a lot of wings look very good. You always need more than two pieces, but Rondo + a player of Howard's caliber would be the core of an elite team.
The rest is the easy part.
Clearly those are two VERY strong building blocks to start your foundation. Greater perimeter defender, Elite interior defender.
Sign Davis, start him at the 4, and he gives you the ability to spread the floor when need be, with his ability to his from 15-18 ft. Paul still starts at the 3, and you have money to sign a SG (maybe even Delonte, who would still probably come pretty cheap, and has shown the ability to start, and I'm sure would love to stay in Boston.)
The rest is speculation. But yes, clearly Rondo + Howard is everything a team needs to build on and gives you a great foundation offensively, and defensively.
There's no question, if he is available in 2013 and doesn't pick up his option, the Celtics (like every team) will make a run at him. And playing with Rondo, and the history of the organization can't hurt. Long as he can deal with the cold weather...