Her biological father is back in her life! What?

I was with my daughter's father on and off for 5 years. We had many problems between us and things just went bad. Well, after Xmas of 2010 my ex and I quit speaking (Daughter was 3 then). With the exception of accidentally exchanging a passing glance at a local Wal-mart once and one facebook conversation, I had had no contact with him (2+ years) until two and a half weeks ago (daughter is 5 now). I had a bogus DSS case open for not even two months and during that time he was informed of it since he still technically has rights. While signing papers closing the case my worker told me she had spoken with him. Apparently he had told her that he'd want to be a part of our daughters life. At that point I laughed in her face and explained why that sounded so crazy considering he had many ways of contacting me and never did. After letting it sink in, I got on facebook, ready to send him a message full of very thoughtful profanities.. but instead I went out on a limb and asked if he really wanted to see her. He got back to me the next day and the day after that I took her to meet her little (half) sister. I'm still struggling to accept the situation and I worry that he'll just hurt her again but I try to feel like it's worth it for her to know her sister. It's insane having him back in our lives and I still don't know how to feel... it's crazy how things work!!