As to regionality, playing a role, well, I live in a place where diversity isn't theoretical, it's a fact of life. But ethic jokes and stereotypes are bandied about pretty casually, albeit in a halfassed, semi-sarcastic manner.
As mentioned in the thread at least a couple times, there is nothing inherently "Jewish" in JAP....you can just as easily (if not more so) point out the Jersey Catholic girls who fit the attitude and wardrobe and spending habits, or the Texan born-again chicks. So I'd be happy if we could just drop the arguably anti-Semitic "J" altogether and just call them "APs".
Certain words cannot be said out loud without setting off a series of complicated psycho-cultural explosions: the N word among African-Americans, the F word among gays; the C word among Chinese-Americans. Italian-Americans have a similar relationship with a two-syllable word beginning with G that is actually a man's name. And their feelings burst out loud when MTV began promoting its new reality show Jersey Shore, which an off-camera announcer declared would feature the "hottest, tannest craziest Guidos" in New Jersey's beachside communities. Wait, did MTV really just say "Guido" on the air?
Most people on the east coast easily recognize the word as a slur against Italian-American men of a certain class and swagger — and there was MTV just letting it rip. As the ramp up to the show continued, Italian-American anti-defamation groups started their drumbeat and the commercial was tweaked ever so slightly: the word "Guido" was replaced with "roommates" — which is more generally the premised cast of the reality show. But that was not the last we heard of Guido, well, because it's all over the show. Indeed, in the first episode of Jersey Shore, the eight housemates wear the Guido and Guidette badge proudly.
The horns thing is truly weird. Katullus, that happened to me a number of times in northern TX and rural PA growing up. I'd be hanging out with other kids and out of the blue, they'd either ask to see them or want to know when I'd had them removed. Two of my roommates at college Freshman year asked if they could see them, and where they were. And as a sign one particular relationship was doomed to fail, a girlfriend's father in high school (South Korean) told me that I was the first Jew he'd ever met, and he was surprised because I "wasn't that bad for a Jew" and I hid "my horns very well." He was all smiles as he said this. He thought he was complimenting me!