Steve,
To continue your off track reply, in Wisconsin we've taken it just a little bit further. We have what's called Recreational Immunity laws. Basically it's the same law that you described but expanded to include any recreational activity. I'd say on average, this law comes into play 3 - 5 times every year for a variety of liability claims that we receive. The last one that comes to mind was from a lady who was sitting in the bleachers watching a fast pitch softball game. The batter hit a foul ball that nailed the lady right between the eyes, which not only broke her glasses, but did a pretty good number on her head too! She filed a claim against us for negligence. When I spoke with her about it, she claimed that we were negligent because we didn't make the fence around the ball field tall enough. Ultimately the claim was denied on the basis of recreational immunity, which she never appealed.
One caution, however. These kinds of laws generally do not apply if the injured person has paid any kind of admittance or participation fee. So in your example, if the injured party had paid to use the property owner's horse, riding area, or had just paid to observe the activities, the injured party might have a claim.