Actually, I was thinking that the whole network of users could be alerted, so people nearby, not just a call-center or 911. As usual, the issue is who to trust and not to trust from the userbase, but it's a problem worth solving - the equivalent of someone screaming for help.
in the UK they recommend that if you need help, you actually shout fire. Sadly in big cities people will 'put their heads down' and walk past muggings. The idea is that if you shout fire, then all those individuals are also at risk, and therefore come to help.
