Google has just announced that they are adding Chat capability within Gmail! Go to http://mail.google.com/mail/help/chat.html to learn more about the specifics. Over the next few weeks, it will be deployed to all Gmail accounts.
Adding chat to Gmail should be a huge boost for Gmail for several reasons. First, it will be integrated right into Gmail's Web-based application, so there will be no need to launch a separate application like Google Talk. This is important because it will simplify its use, and nicely integrate Gmail with chat. Second, you will not need to rely on cryptic screen names as is typical with most chat applications. You will be able to use the verbose naming that Gmail's Contacts uses.
Additionally, you will be able to set your availability status, and see when your friends are online allowing you to decide how you will contact them: by Gmail or via IM through chat. You will also be able to optionally maintain a history or log of your chat sessions.
I'll post additional information as it comes out!