Roaming across a MAN
As more people discover the advantages of wireless high-speed network access, they want to remain online all the time. 3G cellular networks provide this, at an extremely steep price (pennies per kilobyte). With sufficient density a wireless MAN could provide the same thing, with some significant restrictions.
Unfortunately, 802.11-based wireless device simply don't have the range or penetrating power necessary to provide complete coverage. It can take several APs per floor of an office building just to provide sufficient signal coverage. Doing so on a city-wide scale is almost totally infeasible. Smaller areas may be able to manage it, for instance there has been talk of making each of Portland's new parking meters a node in the MAN, which would easily provide enough node density to completely blanket all of downtown with coverage, at least along the streets and along the outside edges of buildings. Roaming in such a situation would be highly useful, as moving a single block may transfer you from one AP to the next to a third.