I dont think it was ever a real issue to be honest. Even with the first batch of "oil spots" people have been cleaning them to where it eventually went away.
With mine, i tested it right out of the box, zero issues. tested it again after 150 shots, still nothing. Im pretty much not even thinking about running the test until its actually visible in one of my photos, and when it is, i will simply clean it.
Cleaning your sensor seems to be something you will want to learn on your own anyhow. JMHO.
Also i wouldnt decide between a D800 or D600 just over oil spots, i would look at each of their features and see which one will suit you best and go with it. Im sure others would be appalled with such mindset, as i was before i just made the purchase.
what the deciding factor for me was that i mainly shoot portraiture type of photography, so i will rarely go above f/11 to notice the spots if they were ever there, 24mp is way more than enough, any larger than that i will be having storage issues...and even at 24mp, i moved to being able to shoot 700 photos on a 16gb SD card to 300...with the D800, approx cut that in half! (Approx) (36.2!) thats crazy..and i moved 10mp higher than what i had before, and it takes a bit longer for the files to download, now imagine with another 10?
again thats just my own concerns, others shooting wild life or landscape/architectural type of photography may be beneficial. I also like the D600 size in comparison to, its dual card slots, function buttons, overall layout as well, as i moved from a DX style body.
the D600 overall regardless of oil spots had me at hello over the D800, especially knowing its about 800 dollars more.
Others, they expect much more, as i do too, especially if im spending 2k dollars, i expect some QA coming out of the factory. But looking back, and actually shooting with the D600 now, it really wasnt much of a concern honestly. Not too sure if the price drop was a result of this..but if it was, win win for me considering i paid 1900 dollars with a lens, prior to , it was 2400 to 2500 dollars. Might just be Nikon's xmas sales, who knows.