Released today, "SwirlySMS free for iPad 3G" offers a free SMS solution that allows you to send and receive messages from your iPad 3G. This is a jailbreak-only solution. Available from the Cydia store, the new app uses your iPad's SIM to introduce native texting without having to hack your way into command-line solutions.
The application should work with any supported SIM that offers SMS features. You will be charged for SMS text messages according to the plan associated with your SIM card. I do not believe it's possible to add SMS packages to the native plan, so if the app works at all, you'll likely be charged $0.25/text at the standard US AT&T a la carte rate.
The app has been tested primarily in Sweden on the Telia network, but its developers have been doing SMS apps for three years now -- so it's a good bet that things should work out of the box with standard AT&T SIMs for United States users. Given that the app is free, if you have any problems, I'm sure the devs will offer a full money-back guarantee.
After thoroughly testing the software with your equipment, you can upgrade to a no-ad version for five bucks, or a full-featured MMS/SMS app (SwirlyMMS) for twelve.