This problem is fairly well documented and primarily affects the Canon 20D and The Nikon D200. Canon attempted to reduce it's occurrance with a firmware fix whilst Nikon's solution was a return to Nikon tweak of the amplifier gains......see Nikon's bulletin here http://nikoneurope-en.custhelp.com/cgi-bin/nikoneurope_en.cfg/php/enduser/std_adp.php?p_faqid=16474
As with all bulletins, the error is "talked down" to avoid mass returns by people who are never likely to encounter it.
Concensus at the time was that the pixel density was set very close to the limits of the technology.
HTH
Bob