Foursquare's site is down and has been since at least 07:07 a.m. ET, as officially acknowledged by Foursquare on its status blog.
An update posted at 8:35 a.m. ET (though the update says p.m., we believe it's an error) says Foursquare is "working to identify the cause of the outage" and promises another update "soon."
We'll keep you posted as soon as we find out more about this outage; in the meantime, it looks we'll all have to put our checkins on halt.