A few of the attendees of the June 18, 2009 NCCPL Tech Group meeting were interested in an iPhone app I mentioned -- Safety Button.
The reviews in the App Store are not overwhelming -- not boo nor yay nor meh. Interestingly, one reviewer tested it, and found that it worked.
It is $2.99, which is not as good as free, but might be worth it to you. I have yet to decide whether I want to invest in it.
I noticed that there is another app called Panic Alarm that works slightly differently, but is $.99. The app has no reviews for the current version.