I think I dated a "rebound guy" once. He had this ex whom he'd dated for, like, 2 years or something, but they broke up several months before he started dating me because she was really unstable (to put it nicely). However, they still talked all the time, and she was still very much a part of his life...even his mom made presents for her (while I was dating him) and stuff like that. The only thing was, he kept all this from me and never mentioned it; I just happened to find out from his friends and other people who knew both of them. He insisted it was over, and I have no doubt it was, but I think he liked the drama of still being in touch with her (because believe me, that gal was a walking soap opera) and in the end that's what made me run far, far away.
I agree with whoever said to watch guys who get too serious too fast...that can be a warning sign of a rebounder. This guy did that, and then kinda freaked because we were getting in a "rut"...hmmmm.