When Facebook announced check-in service Places almost two weeks ago, they also extended the Graph API to allow developers to access check-in data. The API has severe limitations: user could only check-in only via Facebook and check-in data included only people within your network (eg: friends and friends of friends). However, Facebook partners such as FourSquare, Gowalla and others were given priority access to integrate with this new service. As part of their access, they were allowed to enable user check-in via their applications. And as of last week, SCVNGR and Yelp were rolled out this check-in capability.
I tested out the check-in process via both SCVNGR and Yelp on the iPhone.