We got stuck in AP too (interview in early April). We were told that our case is pending AP and that we'll be contacted via phone/email once they're ready for us to send them our passports. The officer didn't disclose anything about the reason why our case was picked for AP, but we were definitely stunned by the idea that we'd have to wait a couple months at best to receive our GC. From what I've read about it it's a common procedure, and usually takes 6 weeks +-, presumably depending how much dirt they can dig up on you (they as in the State Dept, FBI, CIA etc). It sounds far fetched but since 9/11 the US has been very leery about allowing people in.
Hope your case clears quickly.