My understanding is that the only way to make that economical is to negotiate rates on a per-carrier basis, which won't happen unless you're providing a service

My understanding is that the only way to make that economical is to negotiate rates on a per-carrier basis, which won't happen unless you're providing a service that a large percentage of cell-phone users are clamouring for (e.g. Facebook or Twitter). Plus, you have to deal with things like handling STOP, anti-SMS-spamming legislation in each jurisdiction, phone number reallocation, cell phone batteries dying, SMSCs that lie about deliverability, SMSCs that spontaneously send messages from 911 instead of from your short code and plenty of other headaches.