On my tour planned for this year I will miss my Golden Retriever terribly, especially when camping, but my plan (and the plan of the group I am riding part way with) includes too much daily mileage to pull a trailer. If I did not plan on longer days or going with a group, I think a trailer with a dog (sharing space with my gear) could make for some super fun times. Plus my dog would likely help pull me up the hills if I were going too slow.
Do you ride with your dog now? Dog's love nothing more than running alongside their people.