I think segways being allowed in Disney would be a horrific idea but not based solely on the operator's good or poor control of them. I was in Disney on Saturday and Magic Kingdom was pretty crowded I can't even count how many times I was bumped into by strollers (at least five times!), wheelchairs, and children (especially if they were wearing heelys! I can't say how much I think those are inappropriate for the parks).
In fact, most of the trouble comes from people, especially small children, suddenly stopping or turning right in front of you so imagine if you will little Timmy getting excited because he suddenly saw Tigger and running right in front of you. That's exactly what happened the other day and I nearly tripped over the poor kid. Imagine if I had a segway going 12 mph (albeit it is the worst case scenario).
And for the many people who actually would benefit from a segway, there will be those few who just use them to tour the parks faster (unless there is a concession made for those who truly need it but how that is to be determined brings about more problems).
My point is that Disney World DOES provide good alternatives to segways and as long as it continues to do so, I can only see disaster if they segways were allowed.