Imagine this incredibly basic app:

• You and your buddies are on the same paintball team
• You all strap iPhones to your forarms (screen out)
• You all have bluetooth headsets
• You are all on a conference call
• You all see a Google Satellite view of the terrain
• Your teammates all show up as dots

• Instead of putting it on your phone, put it on your gun
• On your map, it always shows what direction your teammates' guns are pointing using the new built-in iPhone compass
• The phone listens for vibration or sounds or something to signal that the gun was fired
• On your teammates' maps it shows little fading lines shooting out from your position, so they see where you're firing. 

• Every time you touch the map it shows where you touch on everyone else's maps.  Simple use, say "rendezvous here" and point where on the map.  Add all sorts of gestures to indicate enemy positions, waypoints, etc.
  
• Add in some kind of external camera and point it where your gun points so you get gun views; share them in realtime with the rest of your team, either always or only when the gun is firing.