Their relevance ended in about 1918. You knew what I meant.
So does the bank on my cheques.
Actually, what I thought you meant was what you said. Excuse me for the factual correction.
edit: Apparently, according to Wikipedia, China, Japan, Mongolia, and Iran use it as well.
The “Real world” I was referring to was the one you and I actually live and go about our daily lives in- ie, a Western, English speaking “Real World”.
You just happen to be right at the moment, but for most of my Straight Dope career, I was in the YYYY-MM-DD “real world.” Still, this is the Straight Dope, after all, and it’s a useful factual point, especially since it seems that, you know, the largest nation on the planet uses Big Endian time format.
They also put their family names first unlike the rest of Europe. And eels in their hovercrafts.
I try to write out the month every time to make it less ambiguous, in MMDDYYYY. I’ve noticed that US Military people seem to write it as 11-MAY-2010, often in all caps.
Personally, I’m still holding out until the French Revolutionary Calendar comes back into style.
How about we leave the metric system out of it for now, and agree that you’re right on writing dates (date first or year first I don’t care, but the month should always be in the middle), but wrong on writing numbers (one period and possibly multiple commas per sentence; one period and possibly multiple commas per number.)?
Yeah, that’s where I get weird again. I’d simply say, “on thirty-one December 1999, we were all hunkered down in our bomb shelters awaiting the worst.”
That is how ISO 8601, which standardizes the format for the exchange of dates and times, does it. The plain date format would have May 11, 2010, as 2010-05-11. I don’t know how well-implemented ISO 8601 is, but it seems to be pretty common across the internet and other computer-based applications.
That’s what I said, didn’t I? I’m fine with changing to the US number style, it’s equally sensible to ours IMO. I’m even inclined to agree with your parenthesis, that the US style is marginally more sensible.
But perhaps I shouldn’t give that away so easily, if we need it as a bargaining chip for the metric system 
I looked at the 8601, but it still has an issue in that it uses the colon for the separator in the time, which is also illegal in a Windows filename.
I’m sure I’d seen a ‘packed’ version that had no dashes or colons as separators.
Edit: yes, here.