Anyone Know a Good SMS Payment Gateway?

The ideal process is the user would put his phone number into the web app, he would receive a text message, "text back <some PIN> to pay $X," and then payment would send a message to the web app server, maybe GETing something like Y.com/payment?from=5555555&secret_key=ZZZZZZZ I'd like to get recommendations on who to use, and it's be great to know if anyone has used SMS payments in their own projects, and how it worked out.