The feature doesn't appear to be live yet, but soon users will have the ability to click on a place name -- included in geotagged tweets -- to view the particular place on a map. Next to the map, Twitter users will see a stream of nearby tweets, giving them a real-time view of what's happening in a particular place at a particular time.
Evan Williams asserts that Points of Interest is not meant to "duplicate the functionality of Foursquare or Gowalla," but instead to "make those services work better with Twitter ... What we really care about is the content happening at that place."
So while Twitter has no plans to get into the checkin business per se, Points of Interest will make it easier for third-party applications to support that type of functionality and it will enable Twitter to aggregate location-specific tweets.