1) my "old guy," Corey, a Thoroughbred/Quarter Horse cross, age 26, was, as of March '02, teaching a blind student to ride. Both horse and student compete with the Kutztown University Intercollegiate Horse Show Association Equestrian Team. I do believe that horses are exceptionally suited for this kind of work, for a variety of reasons. Like almost all other situations where horses are working with people in therapeutic contexts, however, both horse and rider are always supervised. A guide horse (mini or not) would, by definition, be supervised only by its guidee, a vulnerable and risky proposition to my thinking.
2) I'd be interested to see the speed trials of a mini horse versus a german shepherd or labrador retriever...the difference is that usually the horse is running *away* from something it perceives as a threat, while a dog generally runs *toward* something it perceives as food. (the predator/prey distinction) In the ancient struggle for survival in the wild, it's only the weak, lame, aged, injured or otherwise compromised horse that succumbs to a pack of coyotes. A healthy horse can outrun the coyote pack which makes me believe that even a mini horse will beat a dog in a fair race. (a healthy horse is more vulnerable to the attack of a single big cat, usually from above)
Just some random thoughts...