I guess I'm in the minority here, but I like dogs. In fact, I like most dogs more than I like most people. Granted, there are dogs who haven't been properly raised and aren't properly penned and they cause problems for cyclists, pedestrians, Jehovah's Witnesses (heh heh) etc, but I seldom have a problem with dogs when I'm riding.
The most effective repellent that I've found is a loud, sharp "NO" as the dog gets close to you. This seems to work 75% of the time. Of the remaining dogs, it seems to me that a lot of them are playing. They're not chasing you to bite you, but to race. Dogs love to chase, and they think you're in on the game. The trick is knowing if they want to play or eat...
Sure enough, there are a few pathological dogs who will try to knock you off your bike and eat you. Be careful about kicking at them, because they might hang a tooth in your sock, then you're in real trouble. I've tried squirting them with my water bottle and that works sometimes. A bop on the nose with a frame mounted pump ought to work, though I haven't tried it.
I guess what prompted me to post were the messages that suggested poisoning the dog - "dog food and cyanide" or "cupcake with laxative". Just remember that nearly every bad dog is the result of a bad owner. It hardly seems fair to make the dog suffer when the jerk that really deserves it is sitting on the couch watching tv.
Sorry for the long post, but I felt the need to rant.....
carl
(speaking for my current dog, Jessie)