Disney is using them at its resorts to get janitors and service weenies to rooms within minutes of a trouble call. Some things Disney does better than anybody else. Cruises, resorts and theme parks is one of them.
These are about the only places that I could see using these things. Besides all of the other drawbacks pointed out on this thread (weight, bulk, limited range, etc.), lots of major cities have either passed or are considering laws banning Segways on their sidewalks. Even here, in amusement parks and large industrial complexes, I have to wonder if golf carts couldn't be just as easily used (as they already are), trading off somewhat increased size at a fraction of the cost.
I just can't see these things catching on with the general public as a transportation solution when you can buy a decent used car for half the price of a Segway.
I think their biggest marketing challenge is to get people to stop referring to them as "these things."