Seen this nice shootout report at a Taiwan forum with plenty of side-by-side sample crops and it would be very interesting to see how a 24MP Full Frame sensor is compared to the 12MP one, both made by Sony:-
http://www.mobile01.com/topicdetail.php?f=254&t=1542849&p=1
(Text in Traditional Chinese, a Google Translated page can be found Here)
It seems that the A850 wins the D700 hands down at the low ISO with its much higher resolving power and the amount of details retained. As for the high ISOes, the tester/author says that the breakeven point is at ISO 800, which is already better than what I expected for the A850.
Enjoy measurbating! Pentax will possibly use the A850 sensor if they are going to make a Full Frame DSLR! I don't think they will use the D700 sensor anymore, unless Sony won't sell them the newer 24M one, or, they need to go really cheap.
Read Also:-
Old Question: Will there be a Pentax Full Frame DSLR?
Full Frame Mini Shootout - D700 Vs 5D