All y'all

Matthew Yglesias is on vacation and is surprised to find out that "y'all" is actually used, and not just a stereotype imposed on Southerners. The discussion in the comments is funny.
I am continually amazed by otherwise intelligent people who make snap judgements on people based on their dialects and/or voices. I have been on the receiving end of many of such judgements, since I have a high-pitched, girly voice that is on the softer end of Texan accents and my speech is littered with things like "y'all" and "like". For instance, a common utterance might be, "Like, y'all need to quit being like 'fuck all y'all' to each other and like, just relax and shit."
Luckily, many of Yglesias's readers know that dialectal differences are no indication of intelligence and in fact argue for the clarity in Southernisms like "y'all".
I've been teased by Yankees and the like for saying "y'all", which is like, the best opening ever for just fucking somebody's world by suddenly turning on the Super Erudite Mouse and explaining, "Actually, y'all is a perfectly logical dialectal invention that compensates for the loss of the second person plural in English. It used to be that the singular address was 'thou' in the subject and 'thee' in the object and 'thy' in the possessive, while 'you' was used as both subject and object while 'your' was the possessive. However, the singular was dropped and the plural address was generalized to both the singular and the plural, causing much confusion. Southerners just subtly correct the situation by using the 'you' address only in the singular and creating the word 'y'all' as a plural address, both object and subject. For the plural possessive, of course, one would simply say 'y'all's', as in 'Y'all need to move y'all's shit before Bubba gets mad. Y'all know how a bunch of crap clutterin' his patio pisses him off.'"
After that, it's hard to act like Yankees have a natural-born superior intelligence.
I found Ken's comments in the comment box to be interesting, since he tries to tackle the linguistically complex use of the phrase "All y'all".

Y'all generally refers to a group within sight of a speaker, but usually that group is small and manageable. For sufficiently large groups, there are multiple y'alls available, such as y'all by the beer, y'all Tide/Tech/Gator fans, etc. The supergroup of this is referred to as all y'all.

Also, all y'all can refer to people not present. Witness "fuck all y'all". This slur is not just directed at the people gestured at, but also at their close friends, their maternal relations, their pets, and various other people, real or imagined, in need of fuckery.

He's making it too complicated. If you understand that Southerners roughly understand that "you" and "y'all" are the modern equivalents of "thou" and "you" in Shakespeare's time, then "all y'all" is easy enough to understand. It's used whenever one might say "all you" or "all your". It has less to do with who is there and who isn't and more to do with just adding emphasis to the statement at hand. Yes, it usually is a way to address a group larger than the supposed audience at hand, but it's not usually a racial or familial address.
For instance, the expression "Well, fuck all y'all" tends to come out only when somebody feels specifically pissed at one person but feels that all his fellows are somehow to blame. Say he pinches you and you turn around and say, "Fuck you," and his friends laugh. At this point, you say, "Fuck all y'all!" It's an emphasis phrase. If it seems complex, remember it's the equivalent of a non-Southerner saying, "Well, fuck all of you!" See? It's simple.
I used to be ashamed of having a Southern accent but in retrospect I realize I probably charmed more men than I would have otherwise by having a lilting Texas accent. And god knows I wouldn't have any type of wit if I wasn't trained to speak in a way peculiar to Texas women--half sarcasm, half good-natured ribbing. So yeah, our way of speaking is a good thing, and trashing on a dialect is ignorant and not cute.