I have tried a number of solutions to haul my 77lb labrador around.

I first tried a BOB Ibex trailer (single wheel with spring suspension). Put a big rubbermaid tub on it for the dog.

A couple of problems.

She shifted around alot, which made the bike weave around. It was somewhat unstable to stop/park the bike. I think these problems could have been overcome with a little more training.
The worst problem was that she would start to whine. I couldn't really "comfort" her (read: rap her on the head to teach her to keep quiet) when she was back there, so she would just keep on whining. She is an endurance athlete extraordinaire when it comes to whining, and she can do it for hours. It actually wouldn't bother me all that much if I were riding alone, but in a group ride, it was a deal-breaker.

Thinking the weaving/instability of the single wheel trailer was the main reason for the whining, I then tried a standard 2 wheeled, 2 child carrier type trailer, I don't remember the brand. Although this was a little wider than ideal, I think it would have worked OK, but still the whining problem persisted.

I then tried a Burley flatbed and put a large wire dog crate on it. Would have worked fine, except for the whining.

I then got a Chariot sidecarrier http://www.chariotcarriers.com/html_...idecarrier.htm
(which also required me to get a new bike since it doesn't fit most road bikes). This worked well, since I could easily reach over and comfort the puppy when she started to whine. She is a quick learner and didn't take long to learn to ride very nicely in this. I did make a few modifications to the sidecar, mainly putting a piece of plywood for her to sit/stand/lie on, instead of the hammock type seat that it comes with. She actually seemed to enjoy the week long, 400 mile trip in this.

There were a couple of problems still, however. First, the sidecar is designed for a maximum of 50 - 60 lbs and the arm that connects the sidecar to the bike would work slightly loose after a days ride. I did reinforce this with some mega zip ties, but still was not entirely satisfied. Never actually had a problem with it, but not as secure as I would like it to be.

The only other issue was the width of the bike/sidecar combination. It's actually about the width of the standard 2 child trailers, but still feels quite wide.

I just ordered a long haul cargo bike from human powered machines, http://www.catoregon.org/hpm/longhaul.htm

(not to be confused with the Surly long haul trucker bike http://www.sheldonbrown.com/harris/surly-lht.html ),

I'm hoping that this will solve all of these issues. The dog will be close enough to comfort if needed. The rack is designed for up to a 200lb load. Its footprint is similar to a standard tandem bike, no wider that a normal bike, so riding along a road will not block the road any more than a standard bike. Hopefully this will be arriving in the next few weeks.