http://www.msnbc.msn.com/[...]338645/

An article on msnbc.msn.com talked about

http://www.mymobilewitness.com/

which allows you to send a text/picture message that can be retrieved by police but not deleted by you under threat. This provides secure evidence against anyone threatening you. Even if you were killed, your assailant would be identified by police. By notifying a potential attacker that you have secured their picture, you may be able to prevent an attack. You can also send a picture of their car/license plate, friends, etc. I recommend reading this to all US T girls/guys. The service is "free" and supported by occasional email ads. It's benefits are obvious.