According to Webmonkey, Google has created an experimental feature to enable sms chat between your friends’ mobile number and you. The feature will be available for all Gmail users from Thursday evening according to the source. It can be activated through the Labs tab in Gmail settings section. I still can’t find the options in my Gmail account, it could be due to the reason that only US users are supported.

With this feature you can send sms to your friends, plus your friends can reply to you directly and you will see it in your Gmail account.

Source [Webmonkey]