You can now send SMS text messages to anyone in your Gmail address book right from the Gmail website.
Go to your Gmail Labs settings and enable the “Text Messaging (SMS) in Chat” feature. The facility is currently available only for US phone numbers.
You can send (and receive) text messages via the embedded chat in Gmail website but not from the standalone Google Talk client. Yahoo! Mail, Windows Live Messenger and AOL AIM service have had the SMS messaging feature for quite some time now.