Watching recent local cyclocross races we couldn't help but notice how many front forks/brakes went by exhibiting a lot of chatter, where the fork legs flex and the brake pads skips along the wheel's rim surface instead of modulating properly. There are a lot of variables involves in trying to solve brake chatter including rim and brake pad combos, toe in of pads, the type of fork, tightness of the headset, and so on, but the most important factor is the inherent set-up of most canti brakes.
So what exactly does that mean, and what can be done to help solve the problem? Lennard Zinn recently did an excellent job of answering these questions so instead of trying to compose our own response here's the link to Mr. Zinn's article - we couldn't explain things any better. If canti brake chatter is an issue for you this is a must-read.