Invisible Mode for Gmail Chat

Sometimes you need to chat with someone about an urgent project, but can’t be bothered by anyone else. And going “busy” just isn’t enough — you might still get that unwanted ping. The invisible mode for the latest version of Gmail chat is available. With this feature, you can appear offline, and still send and receive messages. If you’re using AIM in Gmail, invisible mode also makes you appear signed out on the AIM network. Invisible mode is one of the most requested chat features.
Source Article: http://gmailblog.blogspot.com/2007/10/code-changes-to-prepare-gmail-for.html