More of an educated guess rather than a completely wild one.
I've read a few forums that discuss the 770 and the wsod gets mentioned so often that is must be more prevalent than just isolated rare cases.

If I just took the Woot example you mention obviously not all of the 8000 woot buyers are going to bother posting on that forum that they got a wsod so the 40 that did are just a fraction of the numbers that actually did get the wsod. Just what fraction they represent is open to conjecture.
Also not everyone who gets a wsod knows that it's a design flaw - they might think it happened because they dropped the device or installed some software or whatever so they're also unlikely to report it on a forum. Furthermore any interest a user may have in a 770 forum or 770 thread would probably die with their 770 so again they may be unlikely to return and report a wsod.

Like I said I'm just making an educated guess based on how often the wsod gets mentioned where the 770 is being discussed. Even from that woot example I'd say it's safe to say it's more common than just one in every 200 units although my guess of up to 20% is probably way too high. It may transpire that closer to 10% of units is a fairer guess but in the absence of any verifiable data then it can be argued any way one wants depending on the position one wants to take.