i subscribe to david pogue's (the tech writer for the new york times) mailing list, and one of his recent articles was about text message spam. he posted the instructions on how to stop most spam for at&t subscribers:
AT&T: Log in at mymessages.wireless.att.com. Under Preferences, you'll see the text-blocking and alias options. Here's also where you can block messages from specific e-mail addresses or Web sites.
he also gives instructions for other u.s. cell service providers - you can read the full article
here.
i think this is handy if you get a lot of spam or have a specific email addresses from which you want to stop getting text messages...