The posters who respond with "never" are simply not looking at the big picture. The performance improvements have been too steady over the past 50 years to think we have come to the relative end of further improvement. There are a few important considerations here...
1) As you near the ultimate limit of performance, the relative pool of ability capable of reaching those levels shrinks exponentially,(Think extreme right side of the bell curve). - Our world population is exploding, so the pool won't change much in reality.
2) As we get closer to those limits, the progress will slow more and more. - We're still seeing improvements with some regularity, so we cannot have reached the peak yet.
3) Our knowledge of sports physiology is improving by leaps and bounds in recent years. For example, just a few short years ago everyone thought lactic acid was a source of fatigue and delayed onset muscle soreness. It was essentially the "gospel" and has now been disproven. - We are highly likely to make further discoveries that aid in training and will make achieving peak performance more likely.
4) Ask Roger Bannister his opinion...
When will it happen? I have no idea...but I'll bet the farm that it does happen eventually.