Today at Facebook headquarters in Palo Alto, CA, the company held an event to show off their new Places product — essentially their check-in service. We spotted in the wild a bit early, but that was only on the touch.facebook.com site (the site built specifically for touchscreen devices). Later tonight, it will be a central part of the new Facebook iPhone app as well.

In the introductory video that Facebook showed off to kick off the event, they showed a bit of the app. It looks like its integration is seamless. There will be a new Places icon in the very middle of the app. Just as with Foursquare, Gowalla, Loopt and the rest, you’ll be able to check-in to any venue you’re at. Or if the venue isn’t in the system, you can add it.

When you check-in you’ll be able to see which of your friends are in the same venue as you. And, interestingly, you’ll be able to tag friends who are checking in with you — this will obviously be huge given Facebook’s social graph.

This functionality will also work on the touch.facebook.com site, apparently.

Facebook is careful to mention the privacy ramifications of this. First of all, you can only tag people you are friends with on these check-ins. Also, by default, your check-ins will only be visible to people you’re friends with (though you can change this to be visible to everyone). You can also opt-out of having friends be able tag you in a location updates. This is found in the privacy page.