Kerry is infamous for his nuances, yet his answers were timed perfectly to fit into the 2 minute time frame allowed. There was no hesitation when he answered. Not just once, but time and again. When anyone hears a question for the first time it takes time to process the information received and then to formulate an answer. Kerry answered almost immediately after hearing the questions. Again, not just once, but almost every time.
Here's my (tin foil hat?) theory. Kerry did get the questions, long before the debate. Long enough before the debate to generally memorize his answers. The questions were numbered and so were his responses. The black pen he removed from his jacket pocket was actually a small bunch of cards with a short synopsis of his "spontaneous" answers (just a reminder) with each card having the corresponding number to the question that would be asked. As each question was asked all he would have to do would be to glance at the card to remind him of the "lines" he had memorized. Perhaps just some sort of shorthand reminder. Next question, next card.
He knew there would be no followup from the moderator and anything President Bush said as a followup could be met with a feint (read lie).
Keep in mind that when senators give speeches on the senate floor they generally have them memorized, and just glance at their notes occasionally.
I'm not so sure this is tin foil. Your theory makes a lot of sense.
1) No real evidence.
2) Too big a risk for a limited upside for both DNC and PBS.
3) Kerry's lack of hesitation has alternate explanations, such as simply practicing his Iraq war talking points, and not being slowed down by concern over the accuracy and sense of his comments.