That pic of the pyr reminded me of something, we had some when I was a kid, the first was Bridget and the second was Bonnie. I think we have an old home video somewhere of my sister w/ Bridget tied to a wagon w/ a lawn chair in it, she's sitting in the lawn chair w/ a little stick tapping Bridget on the butt trying to get her to go, I'm sure it was alot funnier in person (Now I'm going to have to find that tape)! Bridget was just sitting there like it was any other day.
Not to suprising, I think she used that wagon w/ the lawn chair in it quite a bit, Bridget and our nubian wether Jack, both got plenty of time pulling our cocker spaniel Lady around in it.
I considered training my danes to pull a cart, but never really went anywhere with it.