Originally posted by todd Something I've noticed too and curious to see if this got addressed with the k3...
Taking photos of subjects approaching the lens is something virtually no AF can do well.
Basically, the AF has to figure out the right object to focus on (in itself a challenge), determine the relative speed/movement between shots (assuming the photographer is shooting in burst mode), then prefocus the lens in front of the object in sync with the next shot, taking shutter lag into account.
What pro photographers do is not take a head on approaching shot, but shoot the subject moving partially sideways (which slows the relative z axis velocity component), and move the camera with the subject, to reduce the relative movement further still. This turns a difficult problem into a much simpler problem for the camera to work on. There are some who deliberately adjust the camera to slightly front focus, but I don't like that approach.
Once you figure out the right approach, it will work on any decent camera, and the K-5 is more than adequate.
Shots from the Sydney triahtlon, shot on K-5 with DA* 50-135: