I think this problem has been already pin pointed with some blogs I've read in the net especially at www.mansurovs.com
D7000 review. As the pros say, this is OIL SPOT that is deposited from the D7000's shutter mechanism. There has been hundreds if not thousands of D7000 owners who have experienced this problem already. I am convinced and decided to return my D7000 and might get the new SONY A77. Let me just also add that before I upgraded to DSLR, I had a Sony Bridge Point & Shoot HX-1 camera and I have never under any circumstances experienced this problem with the sony even shooting under extreme winter snow storm conditions. All images are speck free from the HX-1.