Marc Spears of Yahoo Sports is reporting that the Celtics are close to a deal with Jason Terry. In 3 separate tweets, Spears reveals this information

Celtics are in final discussions on terms of a multi-year deal with Mavericks free agent guard Jason Terry, sources tells Yahoo! Sports.

Keep in mind Celtics coach Doc Rivers said he wanted both Ray Allen & Jason Terry

Jason Terry expected to land a three-year deal paying the mid-level exception from the Celtics, sources told Yahoo!

Marc Stein of ESPN confirms the Spears report:

Confirming @SpearsNBAYahoo tweet, source close to situation says ofJason Terry's talks w/Boston: "There's a leprechaun in his window."
— Marc Stein (@ESPNSteinLine) July 3, 2012

But, let's not celebrate just yet. Stein added this little bit of a buzz kill:

But source stresses that Terry will go back to Mavs to give them chance to counter before finalizing three-year deal, believed to be at MLE
— Marc Stein (@ESPNSteinLine) July 3, 2012

This is very good news.Terry averaged 13.8 points, 3.8 assists and 2.3 rebounds per game in 34 minutes per game for the Mavs in the playoffs this past season. He brings energy off the bench and can score.  He's also able to handle the ball when Rondo takes a break.   Of course now we have to wait for Mark Cuban to decide if he wants to counter that.  Hopefully he doesn't and another piece falls into place for the Celtics.  If Ray doesn't come back,  this would make his decision a lot easier to deal with.

UPDATE:
In a story on Yahoo, Marc Spears reports that Terry and the Celtics have reached an agreement and mentions nothing about allowing the Mavs to counter. Sounds pretty definite to me. Spears also mentions that Terry has expressed disappointment that he wasn't offered an extension after the Mavs won the championship in 2011. Sounds like Jet to Boston is pretty much a done deal.