Thank you all for your compliments.
The shots were all taken probably about 5-10 feet from the dogs. I've been pretty impressed with this lens so far, especially at the price point. I used it on my past D40, and it worked just fine but manually focusing (especially with action shots) was quite the frustration...
There's talk of a 50mm 1.4 AF-S for the D40 owners, but I'm sure it'll come at quite the price in comparison...