Forget the "dog repellent" sprays and ultrasonic noise makers. The ultrasonic units are bogus, and the "dog repellent" sprays don't contain enough pepper for encounters with two-legged varmints.
I always head out the door with standard, "cop strength" pepper spray in my right jersey pocket. It works incredibly well on dogs - stops them in their tracks nearly every time, and doesn't seem to cause them any undue pain. It's also nice knowing it's there in case of a road rage encounter (FWIW - I've never had to use it on a human). Depending on where you live, you can purchase pepper spray at gun stores, hardware stores, lock shops, and via the web.
See
http://www.crazyguyonabike.com/journ...20040118184701 for a lot of good info on dealing with dogs.