"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. So, as some people have already noticed, we rolled out invisible mode for the latest version of Gmail chat last Friday. 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 and so we're happy to add this option, starting in Gmail Chat, and in the future all Talk clients."
I see "invisible" as an available status in my web instance of Norse Chat, but not yet in the Google Talk desktop app. Perhap it will find it's way there sometime soon too.