Official Gmail Blog: Really new in Labs this time: SMS Text Messaging for chat:
"How often do you try to chat with somebody and they don't respond because they just walked away from their computer? Or maybe you're in the middle of chatting with them just as they need to leave. But you still need to tell them something -- something really important like you've moved where you're meeting...or ice cream! We need ice cream! This is why we built a way to chat with your friends even when they're away from their computers. Now you can keep the conversations going with a new Labs feature that lets you send SMS text messages right from Gmail. It combines the best parts of IM and texting: you chat from the comfort of your computer, and your friends can peck out replies on their little keyboards."