I carry a pepper spray, ordered through a neighborhood watch program, when I hike. I have thought about carrying it somehow on my bike; not just for dogs, but I cycle alone a lot too. Since I fell and consequently rolled over on my back a few times, I don't think the pocket of my jersey would be a good place to keep it. That would have really bruised my ribs!! It needs to be easily accessible though. If you have a Road ID http://roadid.com/Common/default.aspx you could clip a pepper sray to it I suppose.

I don't dislike dogs, but if it's them or me, they're going down!! Now a cat, that's a horse of a different color