The Segway is the much-publicized compact, self-balancing, battery-powered scooter. It is said to attain speeds of 10-12 mph. Actually, according to this page, they can go up to 20 mph but are artificially restricted to lower speeds. (I wonder how long before teenagers discover how to disable the governing mechanism?)
There is a debate underway in Boston, and likely in other cities, as to whether they should be allowed on sidewalks. According to a 7/12/2002 Globe article, "A proposed bill would give the 83-pound devices access to every Bay State sidewalk, trail, and low-speed road, and would bar cities and towns from imposing any restrictions on the Segway." WalkBoston opposes it.
It seems to me that one of the things bicycle advocates should try to find out, soon, is how well Segways and bicycles mix. My own feeling is that Segways do not belong on sidewalks any more than bicycles do... but that Segways add to the rationale for designing lanes specifically intended for "mid-speed" travel. That is, Segways belong in the bike lane... and that's another reason why we need good bike lanes.