roadbuzz,
It isn't just the finish that determines whether the race will be a bunch sprint, but also what comes before the finish. Are there enough hills to allow the other riders to drop the sprinters by enough to stay clear to the finish? That has been one of the interesting issues with this event: The Poggio allows riders to get a small gap but usually not enough to hold to the finish.
I believe that the difference this year was that Bettini went over the Poggio with a teammate who was willing to bury himself to get Bettini safely to the line, knowing that he would then "settle" for third. Had these three riders been from three different teams they very well may have been caught because they wouldn't work together well enough.
And yes, Bettini has an incredible sprint for a non-specialist.
When the "run in" to the finish is totally flat for miles, the teams with sprinters will dial up the pace so high that nobody can possibly attack. If the group had come over the Poggio together you would have seen that this year, as we usually do in this event.