No, the Celtics don't have an open roster space to sign Lue as a PG, but according to the Globe, they have signed him as Director of Player Development. There have been many Celtics fans advocating the signing of Lue as the back up PG. They could certainly do worse (see Marbury, Stephon and Cassell, Sam).
The Celtics currently have point forward Marquis Daniels and shooting guard Eddie House who fill in at the point as well as a young PG with promise in Lester Hudson. They also have 15 players under guaranteed contract. According to the article, Lue will "work with the coaching staff and observe practices." I'm thinking that this is insurance in case they decide that Daniels is more needed as a defensive 3 instead of bringing the ball up and setting up the offense. And just in case they realize that Eddie is much more effective as a spot up shooter instead of having to run the offense. And in case they realize that Tony is much better on someone else's team. In that case, they have a PG in the stable who already knows the Celtics system and can step in and back up Rondo at any given time.
The article says that Lue isn't expected to resume his playing career, but I wouldn't bet on it. The fact that Lue and KG are best friends and that Lue had a hand in convincing KG to come to Boston more than likely helped to sway the decision to hire him. He is a quality veteran with championship experience. Whether he eventually slips into the back up PG spot or he is insurance just in case, or even if he just works with the players in practice, it was a very good move.