Resident date format pedant here.
Like Bud said, yyyy-mm-dd has nothing to do with the U.S. It is the international date format standard:
ISO 8601.
Most Americans use mm/dd/yyyy because it's tradition here (no idea when or how that got started). As is well known known to the rest of the world, we Americans tend to be creatures of habit who see no need for unambiguous international communication, or international standards of any sort.
Despite being born and schooled in the U.S, I hate the U.S. style (mm dd yyyy). The rest of the world (just about) uses dd mm yyyy, which makes more sense. Unfortunately, the existence of the U.S. format makes both formats ambiguous, so neither one can be used -- unless it's inconsequential (or even desirable) that U.S. residents misinterpret it. Particularly on a board like this one, where most of the members are North Americans, using dd mm yyyy would be a recipe for disaster.
The ISO 8601 format is the only readily understandable alternative without resorting to spelling out months. That's why it was in use.
The sooner Americans are shamed or tricked into abandoning their preciousss date format (and English units of measurement), the better, as far as I'm concerned.
There's an interesting graph of international date-format prevalence at
http://en.wikipedia.org/wiki/Calendar_date#Usage_issues
BTW, the default vbulletin date format is the U.S. style: mm/dd/yyyy. I have no idea why. I thought most of the developers were British.