According to the Google Blog, shortly all Gmail accounts will have a new feature that enables the account owner to see exactly where their mail account is logged in and available from. One of the things I love about my Gmail is that it remembers my email login making it easy to check mail in just a few keystrokes. Unfortunately I’m very nervous when I’m at a public computer like I was on Friday at the Apple store in New York. If I forget to log off — the person who uses the computer next will have full access to all my company email. Starting soon (it will take a while for all Gmail accounts to have this feature) - you’ll see a link at the bottom of your Gmail (right underneath where the system shows you your storage space). It will say “This account is open in x other locations. Last account activity: x hours ago”. Then you’ll have an option to see the details and if you want to close out all other sessions.

Gmail Blog - Remote sign out and info to help you protect your Gmail account