Look at this series of articles by Chipworks:-
Full Frame DSLR Cameras Part I – Nikon vs Sony
Full Frame DSLR Cameras Part II – Canon stays the course
Full Frame DSLR Cameras Part III – New entrants and look forward
Last Related:-
Official! Full Frame Pentax System is Coming!! (Latest Interview with Pentax Ricoh Head)
Twelve Years of 135 Full Frame Digital Cameras (Timeline 2012->2000)