Google first launched this feature in October, but was quickly removed due to bugs. Now you can use this feature in your Gmail Chat. You can send SMS Messages to anyone right from the chat window. Just enter the phone number to which you want to send SMS Message, then the recipient can reply you by simply selecting reply option from his phone.
According to the Gmail Blog, the service is currently applied for the US numbers only, you can send SMS Messages from anywhere in the world to the US numbers only.
what Google says about this feature..
Text Messaging in Chat
Send SMS text messages right from Gmail. You chat from your comfy computer and reach your friends on the go; they get your messages as texts and can peck out replies on their little keyboards.
To activate this feature in your Gmail account, simply click on Settings, and then click on Labs tab. Here, scroll through the list and find the option "Text Messaging (SMS) in Chat" and select Enable and Save Changes.