Moral Question, with Ex Boyfriend?

My ex cheated on me about 3 weeks ago with another girl, who he is with now. He has expressed his sorrow to me about a million times now, and truly wont shut up about it...we were very close and have decided to maintain contact. We joke about his "new girl" pively aggressively sometimes, and things are pretty good between us. I have a small amount of lingering resentment, but none that wont be remedied by time and experience. I have had a rebound (one night stand), and he wants more out of it. I am not sure I am ready to move on yet from my ex, which leads me to my question. Should I hook up with my ex again? he has expressed interest in "hanging out" and "catching up". And lord knows I want to. My only concern is his current "girl" will be displeased...like she has any right to be. Any advice?