I think it might have been someone whispering that to *Russert*, to remind him to mention that Reagan raised taxes in his question. Here's the transcript:
http://www.cfr.org/publication/15325/Notice how in the initial question, Russert just asks "will you do what Reagan did with Social Security...." without mentioning what that was. He was probably supposed to include that mention of what exactly Reagan did in his question, so someone whispered that to him as Romney was starting to answer the question, and he interrupts Romney to complete the question.