There is a new free Andriod app that may be of interest to solo and adventure riders called 'CRAsh Detection And Response'. If the phone detects an impact, it sends a message and GPS location to a user defined phone number.
Download from Market.
The delay, before calling, is also user definable; an alarm sounds until it is cancelled, movement is detected or the call is transmitted. GPS has to be turned on, which uses more battery. I presume that cell coverage must be available as well, which would limit its effectiveness in remote areas