Locations, via the Foursquare database can now be shown as part of your tweet, twitter said today.
You can choose to add a location to your Tweets using Twitter for Android, Twitter for iOS, twitter.com or other mobile applications. For example: To give additional location context to your Tweet, you can add a general location label such as “SoMa, San Francisco." In select locations, you may also label your Tweet with the name of a specific business, landmark, or other point of interest. These locations are provided by Foursquare. Twitter Now Allows Adding a Location, Via Data From Foursquare, To a Tweet [support.twitter.com]