You can file it, but I don't think the contents are archived at all. I beleive all they keep is number and date/time.
BUT, if you set up service at http://messages.att.net/ IN ADVANCE, from then on the texts are accessible on the web, but you don't need a supoena for that. You either did it or you didn't. (I don't know how long they stay, but I don't know anyone who does their texts this way so it's going to be a pretty limited number of people who would have this.