How about tactical coordination and mapping for teams (i.e. paintball / special operations)? I'm picturing the HUD 'map' from every FPS game, except it is based on the current view and relative locations of the team's devices.
That would be absolutely incredible... Firefighters die every year because they get disoriented and lost in zero-visibility conditions. Even if it didn't have a pre-generated map of the structure loaded... it could build a map as it went, and at least be able to provide a 'retrace my steps' view.
It could also be networked with other devices, such that all of the routes mapped could be composited and help in the identification of alternative routes should one become blocked.