This could be an upcoming upgrade to Facebook Places, the mobile check-in system available for select countries. You create an Event, give it a location, and when people attend it, they can check-in, notifying everyone that they have come.
AllFacebook mentions that a Facebook spokesperson replied in an email saying that they are currently testing this, and it will be available in the next iPhone release. It is not known when this feature will make it to other platforms, if it will be available to countries that don't have Facebook Places, or what else you will be able to do with it.
I guess this will be something that will give more certain data to Facebook's Friendship Pages. When two friends check into an Event, their Friendship Page will be able to display that they both attended the event. This will get rid of the uncertainty that exists right now. Currently, the Events section on a Friendship Page currently displays only those which both of them agreed to attend. But one (or both) of them may not have actually made it.
Surely, the Event check-in feature will be a great upgrade to Facebook's current features. But who knows what people will think about it?
[AllFacebook via Fast Company]