Find me a DDR2 IC chip rated for more than 400Mhz operation by the manufacturer. I am sure you won't find any.
Manufacturer of memory sticks like Corsair, OCZ etc... buy those 400Mhz IC's. Test them, those that pass their test at 1066Mhz are put in PC8500 sticks but rated for a higher voltage than when they are in PC6400 sticks of course. Hence they are rated to be run overclocked by the manufacturer.
The SPD on those sticks end at 400Mhz because Jedec didn't approve higher speed. That is why they came up with EPP (SLI memory like the marketing jerka$$ like to call it). EPP use the empty bits on the SPD chip to specify tighter timmings and higher clock speed on those sticks. If you don't have an EPP supporting mobo AND EPP supporting ram then you must set the speed manually in the BIOS like in the case of my OCZ Titanium VX2 PC8000. This kit don't have EPP settings but are still rated PC8000 but must be set manually.
I don't have any link to prove what I say but it's pretty common knowledge. I am sure other people can back up what I say.