All Facebook has found that Facebook has begun testing a check-in button for event pages accessed by iPhone users. The check-in icon only shows up on the day of the event and only when users are geographically close to the specified address.
Facebook has had the ability to check-in via their Places service for several months now (though the feature is still rolling out worldwide).
A spokesperson for Facebook explained in an email, "We are currently testing this and it will be available with the next iPhone release."
For the time being, it will be iPhone and touch.facebook.com-only, though you can assume that Android will gain the feature down the road as well. This has the potential to be a great feature because Facebook Events are so widely used. And letting your friends know you are actually at a party you RSVP'd for adds an important location layer to the experience.
It also opens the door to other cool possibilities for events.