They aren't looking too good right now, but I do have my fingers crossed that they are playing dead like they did in the 2009 season, before switching on for the playoffs. Probably not though!
As far as tanking the season, I can't really see them doing that - there is too much pride in the big three. The only way to do it really is to trade one or more of them. I guess if any GM has the ruthlessness to do that it is Ainge, but I don't think he will, because he doesn't HAVE to. Its not like KG, Ray and Jermaine are on long term contracts and the Celtics are going to be in cap hell for a few years.
Let this season play out and then there will be a lot of cap space to work with (about $24 mill, $28 mill if Bass declines his option). They can then have a crack at Howard or any other franchise level player that's out there. However unlikely it is worth having a go.
Worst case scenario is that Ainge doesn't win the sweepstakes. If that happens, he can just go into next season with a roster of cheap journeymen to support Pierce and Rondo and take a nice high draft pick the following year. If Pierce doesn't want to stick around for the rebuild and wants to be traded to a contender, Ainge could oblige him and hopefully pick up a younger player and a draft pick.
To sum up: let the big three have one last crack this season, throw everything at Howard or similar next off-season and go into rebuilding mode if it doesn't come off.