Cingular actually has a device like what you're talking about. Don't ask me what model it is or anything, because it's some custom dohickey they use around here on Capitol Hill to notify blackberry users of potential threats.
It doesn't use PIN, but it uses their xpress messaging service, which allows you to send to
and receive it instantly (without going through RIM or the traditional SMS gateways).
Maybe a gander towards that direction might help you? Or give new ideas on what you want to accomplish.