If you were raring to play around with, you'll have to wait a little longer.
On Friday, one of Gmail's product managers, Leo Dirac, posted on the official Gmail blog that a last-minute glitch has resulted in the feature being pulled back for further testing. The glitch, which Dirac says should take about two weeks to fix, would keep the two-way communication feature from turning on all the way, keeping replies from making it back to you.
When the process is working properly you're able to send SMS contacts a message just like you would if they were online. The system would also keep their responses in the same chat window, making it a simple resource for staying in touch even when you or your contacts are not online at the same time.