It's not even loading the status-page. If you go there directly it only says:
That really sounds like a major hardware problem or a hacking attack. Now many user will have some fear about their personal data and location-based information, I'll guess.
I hope we will soon get a statement on what, why, and when this incident happened. Nonetheless, I like foursquare and can't wait until they are back to check-in again.
Update: Foursquare is back up again. Questions remains.