Back Illuminated Sensors allow for a more effective use of pixel area.
*UPDATE 2*
Another source highly doubts February 7, 2012 is the date for an announcement. All the little bits of information that are leaking say it’s close, but it may not be that close. Please do not treat February 7, 2012 as fact until I can [CR3] it.
caLLous said:*UPDATE* New High-End DSLR Later in February [CR3]
CR3 bitches! It's filed under 5d3 and there's a pic of of a 5d but not much about it in the text. Final release of 1dx samples is interesting though.