My advice to you if you are going to this Q&A - read up on YC first.
It was amazing how many of the questions asked in the Boston session are easily answered by quickly looking at the YC webpage or reading the "What happens at Y Combinator" pg essay.
Maybe that's ok, since it's probably more about meeting people in person, rather than being a formal Q&A event for newbies (I imagine most of the people going found out about it here and already know most of those answers).
If you've read up on YCombinator, go an hour and a half late. I was at the Boston Q&A last night. Nothing I didn't already know was brought up during the Q&A. The real value is the conversation after the Q&A session is over.
It was amazing how many of the questions asked in the Boston session are easily answered by quickly looking at the YC webpage or reading the "What happens at Y Combinator" pg essay.