I really see no advantage or incentive for the Celtics to extend him before the end of the month deadline unless it is at, what they consider to be, well below market price for him. There's just to many different variables at play here that make this a much smarter play for the team if they play a wait and see game. Consider these variables:
1.) Rondo's game - given how inconsistent he can look from game to game and series of game to series of games, is it really unreasonable to think he could have up and down years as well? If he has a down year but they still think he is the player they want, they could lock him up at a bargain price after a relatively not so great year.
2.) His maturity and difficulty level both on and off the court - no matter how he plays this year, Rondo's ego and ability to be coached could become almost toxic and the team could be afraid of this. Waiting another year and seeing who a stil VERY young player grows up some could go a long way to deciding if he is a guy they want to invest in.
3.) The economic climate - the salary cap is going to fall off this year, maybe a lot. By next summer we might be finding out that the cap for 2011-12 might be taking another big hit. Given that in two years the salary cap could be under $50 million, it might be a wiser move to trade Rondo next year for picks and youngsters to try to keep the team out of financial ruin while the lean times hit post Big Three
4.) The impending work stoppage - no one knows what the 2011 CBA is going to look like and it could be that teams will be tip toeing around giving out large extensions if the lockout/strike could change the way the cap, max salaries and othe stuff is handled.
5.) The free agent market of 2010 - a lot of team are betting big time that big time free agents will be available next summer. But if LeBron, Bosh, and Wade al resign with their teams there could be a lot of teams looking to fill the void with a player like Rondo and could mortgage their future for him. It might be wise to play it otu ans see what happens.
These are just five and my guess is Danny Ainge could come up with a lot more. I don't see they extending Rondo unless it's at a dirt cheap price. there's just too many really smart business reasons not to do it.