iPhones, like most other data-enabled phones from AT&T, have a built-in email-to-text address:
Sending a message to this address (replace "yournumber" with your line of service number -- full number including area code with no dashes and no preceding "1") will result in a text message arriving on your iPhone through the built in SMS application. Text messaging charges apply.
Note: For unknown reasons, this feature may not work for all AT&T subscribers. If you receive a bounce-back, try calling AT&T's technical support for assistance.