As with a lot of technology, current laws do not cover this. Also, they (Segways) are new enough to be very expensive for the average person, and are not yet an approved medical assistance device, so they aren't covered by insurance.
I'm just saying that I've lived longer than the average poster, and have seen more than my fair share of changes. The arguments being used are similar (but not exactly the same) as those used against service animals, for example.
People complained that they were dirty, could bite people, could carry disease, etc so shouldn't be allowed into public places, esp restaurants and grocery stores. Then, after enough lawsuits, only seeing eye dogs were allowed, as, at the time, they were the only "trained" service animals. This prompted more lawsuits, until we have today's laws.
The same can be said for Segways, or any other device that enables the disabled to function more "normally." The laws and guidelines should start being written BEFORE they are classified (if it really does happen) so that people have to be trained and certified in their use, just as with service animals. If I were to lose my eyesite tomorrow, I couldn't just put a harness on my dog and walk into a restaurant, or even get a trained dog and do it. I would have to learn how to use him/her safely.
I also have a problem with just anybody being able to hop in ANY motorized vehicle (including an ecv) and drive off without proving at least a rudimentary competence using it. That's where the current law has failed; ECVs were grandfathered in with wheelchairs, rather than having separate guidelines for their use. When I brought my mother to WDW, I did NOT allow her to use one (she wanted to), because she did not use one at home. (She used a walker) I used common sense, and pushed her in a wheelchair. Did it limit her experience and tire me out? Sure, but it spared many other guests cuts, bruises, and scrapes.