It can be harmful for a developing dog to be forced into repetitive motion - the younger the dog, the greater the danger to developing tendons, ligaments, and joints. And exercise can be forced, even if a puppy is willing to jump up on the thing, because once he's up there and it's turned on, he is forced to continue.
Here's another thread on treadmills and puppies that may be of help.
http://www.rottweiler.net/forums/pup...l-puppies.html