It is a combination of the ability of the lens and camera's focusing system. Even if you have a top of the line focusing system, that wont make up for a lens with slow or poor auto focus performance.

From my limited experience, I'm going to agree with NSX While I was very happy with the overall performance of my D5000, I felt that I was losing BIF shots because of the focus tracking. Because of that, I picked up a used D7000 off of ebay and while I'm still geting used to it, I have noticed a definite difference and improvement in tracking birds.

I think it's the body. The lens can only perform so well as the sensor module can track.

With the D40 I only reliably depend on the center one, which really pares it down nicely, as there's only 3 AF points.

Also, whether or not the focus point is cross type or not determines how well it can track movement too.

Has anyone had the chance to compare or notice a difference in the focusing between bodies? I hear some complain about the D7000 focusing. I have noticed my D5000 has trouble tracking sometimes. Is hunting always a function or the lens or body and lens?

I was curious if a D400 is released if the 51 point AF system is really superior to that of the other bodies? What other factors are there (assuming same lens and light conditions). Mostly looking if someone has experience with a lower level body compared to one of the higher level bodies.