This is contingent on everyone - that includes Danny, Doc, KG & PP - deciding that this team has run it's course as currently constituted.
Celtics send: Paul Pierce & Kevin Garnett
Lakers send: Dwight Howard & MWP
Why the Celtics do this?
As mentioned before, if everyone realizes there are no tweaks that can lead them to a championship, it might be best to break up the band.
I know most people here won't be able to stand the thought of Pierce or even KG in purple and gold, but if we're going to move them I don't want to send them to NBA Siberia (i.e. Sacramento). Paul's an LA kid and KG has his home in Malibu. If they're okay with this, then I'd be okay with this.
Now on to what the C's get. While Howard might be a total Summer's Eve, he still is the best center in the league. You team him up with Rondo, AB, Sullinger and Green, you have a talented and athletic (maybe Sully not so much in the athletic dept.) core to move forward with.
The concern with Howard is that he'll be a free agent at the end of the year and he won't resign in Boston. I say if you can bring him in and let him play the remainder of this season with Rondo, he'll fall in love with both the tradition and the easy buckets Rajon will feed him.
Why the Lakers do this:
This current Lakers cocktail just hasn't mixed well. Howard has never quite fit in with Kobe and now D'Antoni. Additionaly, with their age and contracts, their best option would probably be to go for it all right now. I think they're a much better team (and fit for D'Antoni's system) with KG & PP. Those two would allow for much better spacing and ball movement, plus add championship grit that would mix well with Kobe. Plus, all their contract would end in a couple of years (not including KG's option year). That would allow them to retool with plenty of cap space.