I've often wondered why Nancy Pelosi would say such a thing. She should have had enough foresight to realize that statement would bite her in the ass.
My original thought process on this was that not many politicians really know everything in the bills that are submitted to a vote. In a nutshell, bills are introduced and sent to committee. The committee hashes it out and sends a final version to the floor. Representatives and Senators have their aides and researches pick it apart and advise them on the bill. If it comes up for a floor vote, they vote yea or nay, based on what they have been told is in the bill and the recommendations of their staffs. If the House and Senate versions differ, then it goes to conference committe, which produces a compromise bill that comes up for a vote in both houses of Congress. So, no, I don't think our elected representatives know what's in every bill.
But I decided to do some research. Pelosi was speaking to a group (I don't remember who and I'm not inclined to go searching for it again) about the PPACA. She was speaking of all the rumors about what would happen and many of those were falsehoods, such as death panels, prices being fixed, limits on what services would be allowed, etc. Her statement that "We have to pass it to see what's in it" was a reference to the fact that the bill had not been passed and people didn't know what they were talking about when they were claiming all kinds of nonsense about it. At the time, the Senate had not passed a final version, so she did not know exactly what it would ultimately look like.To paraphrase her true intent, she was telling the group to whom she speaking that, until the bill is passed, you can't know what's in it and to ignore all the rumors flying around until it was passed.