Twitter: Tweet With Your Location
(This additional service is now being rolled out across the world)
Twitter’s Tweet With Your Location feature allows you to selectively add location information to your Tweets. This feature is off by default and you will need to opt-in to use it. You can find out how to start using this feature on the How to Tweet With Your Location help page; this article is an overview of the feature.
Once you’ve opted-in, you will be able to add your location information to individual Tweets as you compose them on Twitter.com and with other applications or mobile devices that support this feature. The location information that is shared publicly can be either your exact location (your coordinates) or your place (like a neighborhood or town).
Tweeting with your place or coordinates can add context to your updates and help you join the local conversation, wherever you are.
Helpful links for Your Questions:
How do I tweet with my location?
This service was first introduced in November 2009 [USA only] via the twitter api, now this is available in users settings (off by default).
Most likey because FaceBook Places has been rolled out globally too.