Nikon really has very fast AF system, but knowing that D300 has 51 focus points, that's not some surprise. But I'm very satisfied with Pentax's AF system. It is a bit slower in low light than of other companies, but that goes mostly for night time.
In daytime, it's very fast. I've been taking pics of some birds fyling, and they came out pretty good focused.
Here are some examples:
Unsharp mask was applied to those, but only mild one (not sure but I think it was Amount 50, Pixels 2, Treshold 0), but anyway, those birds ARE in focus. All with kit 50-200, first with center AF point, 2nd and 3rd with multi (and cropped from original). Focus was in Single mode an all 3.