dhackett,
As far as I know, in order to use FronlineSMS (or the similar product RapidSMS) in the United States, the user still has to lease a short code through a SMS messaging , marketing, or management company. Early in our SMS research, we considered using one of these open source SMS platforms, but it didn't take long before we realized that these systems are technically demanding and not as "free" as they might seem. True, the software is free. But the technical expertise required and the SMS vendor are not. And given the level of service that many SMS management companies offer, FrontlineSMS is un-necessary.
I think FrontlineSMS has more appeal in developing countries with less regulated cell phone infrastructures. The USA is famous for having a tightly controlled cell phone system that limits consumer choice and access.
That said, if you find a way to use FrontlineSMS here in the US in a cost effective manner, then I'd be very interested to hear more.
Adam