I think he will be the first to go sub 9.5 for 100m and sub 19 for the 200

100m:

Bolt covered the distance in 42 strides.

If my maths is right (and it often isn't) that equates to almost 8 feet per stride and probably somewhere over 9 once he hits full speed (bearing in mind his strides would've been much shorter after the start).

Astounding.

How long before another athlete of this standard comes along - there must be a limit to mechanically how fast the human body can actually run?

I'd struggle to long jump that!

Average stride for many athletes who run longer distances is around 9 feet, sprinters have to make sure they don't over-stride as this reduces their speed. Bolt is so fast because he can maintain high leg speed and a larger than average stride. It was bound to happen that a freakish tall guy with that leg speed would come along - he's it.

Amazing is definately the word (and clean as well - unlike FloJo who did similar things in Womens sprinting).