According to the latest Gmail Team blog (article here), this feature will allow you to see other logged-on sessions from within your Gmail account. Then you can simply find the session you forgot to log out of - and, well, logout ;) There's also the option of signing out of all except the current one - how handy!
According to the Gmail team, the feature is being rolled out to the latest version of Gmail that runs on IE7 and Firefox. I just noticed this in mine - have you?
Image credits: Google mail