the author of this app says:
textPlus - brings text to the iPad! It's the ONLY FREE, UNLIMITED & GROUP TEXTING app for iPhone, iPod touch & now iPad! More than just app-to-app messaging, textPlus lets you send and receive texts with friends on ANY phone. Now in North America, text Canadian numbers for free. International texting is ALWAYS free when going app-to-app.
Requires iPhone OS 3.0 or later. Wi-Fi or 3G connection required.