I think we have to assume that Nero and his crew were indeed incarcerated by the Klingons for 25 years, in order to explain why they weren't seen for all that time. And besides, there is a brief shot of Nero actually on Rura Penthe as a prisoner in the scene where Spock Prime mind melds with nuKirk. Of course, we then have the rather awkward problem of what the Narada was doing at Rura Penthe when Nero broke loose...