I got my IPhone4 a few days ago and have installed a few apps. The FaceBook App for the IPhone4 enables you to "check in" with your current location like the 4Square.com idea. Mine was working then it stopped, I tried a bunch of stuff. I tried turning off my IPhone, reinstalling the FaceBook application and so on. Finally I found a solution to the stupid problem.
What worked to fix FaceBook check in :
- Go to settings>
- general>
- reset>
- reset location warnings
Other people have had success getting check in working this way:
Go to Settings -> Location Services -> Turn FB on
If either of those solutions don't work you can check here.