Gmail has introduced a new privacy feature that will let users see how many computers their account is open on, and also allows them to sign-out remotely. Basic information is displayed as part of the page's standard footer, and users looking for more detailed information can view a log that displays the most recent IP addresses to access the account, along with the type of access (Mobile, POP, etc.)
The new feature will be especially useful for monitoring email accounts for privacy intrusions, as well as for users who like to use Gmail from public terminals and may forget to manually log-off. Google says that the feature is being rolled out as part of the latest version of Gmail, but it appears that not all accounts are active (I couldn't access it from my account).
You can read more details at the Gmail Blog.