John, do you have a suggestion re good lens for dog agility stuff? Or even something that'd be good for capturing her jumping for frisbees in the park?
Outdoors you have a lot more options. It's just indoor agility that you're going to have a lot of problems with. Outdoors in good light you should be able to get the shutter speeds you need with a lot of lenses. The key consideration is a lens with enough focal length. Secondary is a lens with fast focus capability. My recommendation assuming you were very close would be the Canon 70-200 f4 ($560) - great lens - sharp, fast to focus. BUT, you're going to have to be close (within 20 yards or so). The Canon 70-300 gives you more reach but it's going to be slower to focus and the build quality isn't as good. If you want quality on par with the Canon 70-200 f4 with more reach then you need to step up to the $1000 Sigma 100-300 f4.
Realize though you're always going to have exposure challenges with your dog in bright conditions. No lens will help that. To overcome that, you either need external flash OR choose when to take photos based upon the lighting conditions (overcast is best).
Indoor agility would be very tough - I haven't seen many examples of people doing this successfully although with the latest crop of DSLRs and ISO 6400 performance I'm sure people are getting better results.