http://www.wrestlezone.com/news/260319-cm-punk-reportedly-blades-himself-during-raw-match
Cena taking the strap seems likely if true...
It seemed pretty obvious during the match although I couldn't actually see him doing it. There hasn't been any backlash against him because of it, so he probably Bret Hart-ed his way out of it. Or else he was told to do it.