i checked my 3100..and nothing visible that i can see when i zoom in.
I have yet to do a wet clean or any type of cleaning to my sensor other than the self cleaning option in camera.
I still might consider the D600 even considering. i did some research, and most have experienced "dust issues" however it seems to be some oil residue which can be cleaned over time. Also saw promising reviews of this going away after some time.
I priced the 600 to other models, you still cant ignore the fact that yes...2k is a lot of money, however in comparison to others..its actually a great deal. i would end up paying 1900 dollars for the kit, D7000 would be 1000, a D800, 2600 dollars...and a D3 or a D4, well thats another bracket lol
i will see what they are doing to clean their 600's and i might take the chance and go with it. we will see i guess.