I think what if generally boils down to is that 1) the sign needs to be posted, and 2) It is limited to those participating (generally). So if your horse went mad at a parade and stomped a child to death you probably will be held liable in most states. A great aide, but certainly doesn't absolve one of any liability...
There are only 4 states that don't have equine liability laws. They are listed below.
Activity Liability Statutes (note that four states (CA, MD, NV, & NY) do not have laws)
Summary of PA law:
These statutes comprise Pennsylvania's Equine Activity Act, which sent into effect on February 21, 2006. Under the law, liability for negligence shall only be barred where knowing voluntary assumption of risk is proven in a particular case. However, the Act provides immunity only where a sign that states, "You assume the risk of equine activities pursuant to Pennsylvania law," is conspicuously posted on the premises in two or more locations.
This is a summary of Texas' law:
This Texas section provides that any person, including an equine activity sponsor, equine professional, livestock show participant, or livestock show sponsor, is not liable for property damage or damages arising from the personal injury or death of a participant in an equine activity or livestock show if the property damage, injury, or death results from the dangers or conditions that are an inherent risk of an equine activity or the showing of an animal on a competitive basis in a livestock show. Liability is not limited by this statute where the equine professional knowingly provided faulty tack or equipment, failed to make reasonable and prudent efforts to determine the ability of the participant to engage safely in the equine activity, owns or otherwise is in lawful possession of the land or facilities upon which the participant sustained injuries because of a known, dangerous latent condition, or if he or she commits an act or omission that constitutes willful or wanton disregard for the safety of the participant or intentionally injures the participant. The statute also requires the visible displaying of "clearly readable" warning signs that alert participants to the limitation of liability by law
Link to the site.
Map of State Equine Activity Liability Statutes