Is there any way to retrieve the check in of my friends using the API for Facebook? I am going to develop an iPhone app and I need latitude and longitude.
Yes this is pretty easy to implement with Facebook. Check out the Checkin api documentation which which will give you the places the user and their friends have checked into, including latitude and longitude. You just need to prompt the user for user_checkins permission. This can also be queried using their FQL language, look at the checkin table. The latest Facebook iOS SDK & sample will help you get started making Graph API's quickly.