apparently a fair few have. Sometimes if a quad core cpu has a bad core the offending core and sometimes another core can be disabled leaving potential for one of the cores to be good. Its also known for good quad core chips to sell with cores disabled to meet market demand.
Last year in October Asus released firmware updates for numorous boards which allowed the unlocking of these cores and there have been success stories
http://www.bit-tech.net/news/hardwar...re-unlocking/1
Now however this abilty has been shut down but Asus has on its newer boards installed a hardware switch which can do the job the same
http://www.overclockersclub.com/news/26171/
I would be interested if anyone here has any of the boards listed in the first link with uptodate bios actually has any disabled cores ( I am not saying enable them as it could lead to instability but you can just disable them again after) or if anyone actually has any of the new boards with the new switch and 890GX chipset.
Apparently there have even been some quad core cpus that unlock to 6 cores