This is what I found on the apple site, you need to be running iOS5
Messages with iMessage
Talk it up with fellow iPad, iPhone, and iPod touch users running iOS 5. iMessage on the large iPad display is text messaging made beautiful — and free over Wi-Fi.* If words aren’t enough, you can send photos, videos, locations, and contacts, too.
Learn more about Messages
*For iPad with 3G, normal carrier data rates may apply.
5.0