Facebook, Foursquare, and Gowalla (along with several other companies) now have a little more competition when it comes to location-based check-in services. Starting this afternoon, Google Latitude users can also let their friends know when they've arrived at a certain place. Google's rationale for this upgrade is that check-ins add context. Now, rather than just seeing a pal is on a certain street, a user can figure out whether he's grabbing a drink at a coffee shop or buying a new vehicle at the adjacent car dealer.