I'm going to have to disagree with Tony. I submitted to YPN and about 3 months after I received the same letter you got I received an invitation to join the YPN. It appears they put you on some sort of waiting list after you submit an application. When they open up beta testing further they evidently invite sites that have applied and which meet the standards they happen to be looking for at that time. I basically took the same attitude towards getting into YPN as I did with DMOZ. Submit and forget, not much else you can do