True sprinting is normally a very short burst of speed (200-500 meters) and is separate from pacing. In road racing the top sprinters come up from the midst of the pack and attack for intermediate goals or time bonuses. The main mission of a sprinter is to remain in the pack and hold on the best as they can with strategies like drafting and team support, in order to conserve energy until the end of the race. When the end nears they start to tactically position themselves for a sprint to the finish line. In races where the pack isn't divided as a result of break away riders, the top sprinters are almost guaranteed a win since the Lance Armstrong types don't have a chance against these powerhouse riders. Sprint specialist aren't really contenders for the overall in large multi-staged races like the Tour de France but do have a good chance of possibly capturing the leader's jersey during the early stages of a tour because of a stage victory or two. The fans also love these guys because they're exciting to watch. The top sprinter in a stage race even wears a special green jersey which tells everyone else that he's won the most sprint points.
Track sprints are different in that the duration of the race is measured in hundreds meters instead of miles or kilometers. This is sprinting at is purest form but normally these rideres aren't as popular or as recognized as world class road racing sprinters in huge world cup events.
The top sprinters like Eric Zabel can push a 55-11 gears to around 47 mph on a flat road with no wind. I believe it's very close to 1500 watts. It's interesting though to note that it takes about 40% more power to go 45mph than it does 40mph and that's only maintaining the speed. You also have to factor in that a rider has to accelerate through that region which I think boarders on super human. Track sprinter Nelson Vails was once measured to produce nearly two horse power. These days everything is measured in watts since it's easier to track and measure.