Here's a video of a C172 doing a roll (about 1:00 in) (
http://www.youtube.com/watch?v=GxNgw8_y0nc ) . It shows that is it possible. Even so, I would not encourage aerobatics in a touring aircraft, and you will never find me doing anything more than a steep turn in the 172 I fly. That video of the P68c loing its wings after passing VNE is word enough against not doing aerobatic manouevers in a tourer for me...