Image Credit: Image by BP, copyright BP plc. High-res version available here.
More here from the Houston Chronicle and the BBC.
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.
SW10
Other resources
Edit: Dedicated response website available here: http://www.deepwaterhorizonresponse.com
Edit: An thread dedicated to the investment implications of this event has been created here (BP: A crisis-play?)
Edit: BP's ROV cams assembled on a single page, thanks to Mr.Contrarian and his www.freesharedata.com site.