After adding him to the training roster, the Pistons officially signed Alex Acker. Chris McCosky has details on his blog:
You wonder where or if he's going to play, but the Pistons have signed guard Alex Acker. They gave him a partially-guaranteed one-year deal that would pay him close to $700,000 if he sticks the whole season, and they have an option for him for next season.
Obviously nothing is guaranteed until the start of the season, but the addition of Acker means the Pistons have a full roster of 15.
Update: Actually, strike that, it's apparently fully guaranteed. From A. Sherrod Blakely:
Alex Acker won't have to earn a roster spot after all. The 6-foot-5 combo guard signed a one-year, fully guaranteed contract with the Pistons. By signing Acker, drafted by the Pistons with the final pick (No. 60) of the 2005 NBA draft, the Pistons now have the NBA-maximum 15 players with guaranteed.