1) Sprint either sells or used to sell an app called Drive First.
2) Check out AT&T DriveMode, DriveSafe.ly, or DriveSafe Mode and others... in the iTunes App Store.
I don?t know anything about any of these and I don?t use them. I simply did a search and founds a few articles about them. There are probably many more. Some of these are one price forever, others are by subscription.
Apple will probably add the option for you to disable text while driving in a future OS release.