User Generated Short Codes: "Mechanically, TextMarks allows you to go to their site and reserve a keyword or words. You also type in the response you want to send out when people send that word in. Then, when that word is sms’d to 41411, the sender gets back the message you want them to. Very simple - and it’s all free for the time being."
Neat. Reminds me of the O2 Escape Pods service (now defunct) which did much the same thing IIRC.