I was chatting with a friend earlier this week and she asked me about this line found on the bottom of my Gmail signature, "Sent from: Cary Nc United States." She thought that maybe I'd sent the message from my iPhone, but I hadn't. It had been sent from my notebook, and works with your Gmail account no matter where you are or the device. This capability was added to Gmail just last week: New in Labs: Add your location to your signature.
When she was asking how I did it my first thought was to make a screencast, and here it is:
Gmail: Displaying your geo-location in your signature from Kevin Gamble on Vimeo.
This uses your IP address to determine where you are and then automatically adds it to your signature. It works perfectly when you are on known networks. It may be 80-90% accurate when you are on the road. For example, hotel network traffic is often routed to distant locations, and it may be showing you as somewhere that you aren't. This is really no big deal though. The geolocation information is added to your email messages as soon as your start to compose them. If you originate a message while on the road and the information isn't right, it's easy enough to correct or just delete.
I hope you find this useful.