9:30pm: This trade is now official. After acquiring the 16th pick, the Mavs flipped it in a second deal with the Hawks.
7:59pm: The Celtics have acquired the No. 13 pick from the Mavericks and used it to draft Kelly Olynyk, reports Adrian Wojnarowski of Yahoo! Sports (Twitter links). Dallas will receive this year's No. 16 pick and two future second-rounders from Boston, according to Wojnarowski (via Twitter).
The Mavericks had been considered a near-lock to trade the 13th pick in an effort to clear the cap hold and create more room to pursue Dwight Howard. Now that the team has the 16th pick, we'll have to see if another trade is coming, or if Dallas uses that selection to choose a draft-and-stash prospect.
As for the Celtics, since they gave up two second-rounders to move up three spots, presumably they expected the Timberwolves or Bucks to select the Gonzaga big man at 14th or 15th overall. If not, perhaps those two future picks are heavily protected, making the cost to move up small.