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>I have just finished an SMS Texting app for a client, and it is an expensive proposition to do if you want control over what is happening. You must use a content aggregator and then be approved by each of the carriers you will be using to carry the SMS messages. If you google FREE SMS Gateways, you will find a few that will permit you to send them, but you get what you pay for.

I'll second that. It's expensive - much more so than you'd expect. It's also hard to directly compare companies that provide this, since they all seem to have their own way of charging for this.