You can turn off SMS: contact AT&T and tell them to disable SMS for your phone number. This is exactly what I've done and I highly recommend it. I save $5/month in texting charges, and I can still send and receive texts for free. Here's how:
1. Sign up for Google Voice.
2. Tell people your new Google Voice "texting" number (and use it for voice if you want).
3. Buy Prowl at the App Store for $2.99
4. Push your Google Voice SMS messages to your iPhone via Prowl. You can do it with Fluid and a script on a Mac.
6. PROFIT!!! (free texting)