UPDATE: As of iOS 5 (expected fall 2011), iPad owners will be able to send text, picture and video messages through Apple's new iMessage application. These are not true SMS or MMS messages, as they can be sent only to other iOS device owners (iPhone, iPad, iPod Touch).
For those not running iOS 5, right now the answer appears to be no. Even if you have a Wi-Fi + 3G model of the iPad, the AT&T data plan is strictly for Internet access.
Using third party applications you can send text messages via the iPad -- but messages send using these third party apps will not sync with your existing SMS messages on other devices.