Image Credit: Image by BP, copyright BP plc. High-res version available here.
Cause not yet known, but given that Transocean say the rig was drilling then I can only assume there's been a blowout. Another possibility may be a problem during testing, though that wouldn't really fit with the 'drilling' statement.
Estimates of 11 to 15 people missing, which would be more than just the drillfloor crew.
Edit: Dedicated response website available here: http://www.deepwaterhorizonresponse.com