Interesting discussion. Thanks for raising the issue. My $.02:

I think this issue really boils down to whether or not a nation has a "national costume" (i.e. costume = distinctive style of clothing, vice "fancy dress").

To my mind the US has no true "national costume", despite the fact that I wear seersucker and white bucks here in the South during summer.

Here's what the first google-hit for "national costume" which deals with the issue says:
http://costumes.lovetoknow.com/Natio...s_of_the_World

"Examples of National Costumes of the World:

Scotland: The knee length pleated kilt is characteristic of Scottish costumes, and is often made of tartan–one clan’s specialized fabric–for even more symbolism. A furry pouch worn around the waist called a sporran and knee socks are other classic parts of the Scottish costume.
Russia: Russian national costumes may be heavily embroidered peasant clothes or the calf-length red wool coat, black boots, and cylindrical dark wool Cossack hat of the Imperial Guard.
Japan: The official costume of Japan is the kimono–a long, heavily patterned silk robe featuring colorful scenes and tied with a wide length of silk fabric called an obi. Sandals, stylized makeup, and an elaborate hairstyle complete the cultural fashion.
Bolivia: Elaborate masks and feather headdresses reflect the pre-Columbian influences in Bolivia’s national costume.
Jamaica: The Caribbean tropics come to life through Jamaica’s national costume with long, flowing, vibrant skirts in stunning colors. Accents include tropical flowers and fruits that are well known in the tiny island nation.
Sweden: Sweden’s national costume is one of the most distinctive, with its white winged hat, white long-sleeved blouse, long skirt with apron, and embroidered vest. Classic craft influences are obvious in this coordinated look.
Africa: African national costumes are different than most other countries’ attire in that they reflect the culture of various native tribes rather than national boundaries. African costumes typically include kente woven cloth in bright, bold colors and geometric patterns that reflect different tribes’ customs.
United States: The U.S. does not have a national costume, but if it adopted one, what might it include? Elaborate hoop skirts may be suitable for Southern states, while Inuit and Eskimo clothes are perfect for Alaska. Hula skirts are appropriate for Hawaii, while chaps and cowboy hats might be found in Texas. The key is that national costumes always reflect the culture, heritage, and individuality of the nations they represent. "

I'm not advocating that this website is authoritative or even completely true, but I do think that it reflects the prevailing general understanding of the issue.

Wearing Scottish National Dress to any event, I think, will be interpreted as professing a tie to Scotland. I understand that many XMarkers divorce the wear of the kilt from Scottish identity or descent, but I don't think this reflects the public attitude about such things. Hence the age old question to the kilted man: "Are you Scottish?"

So, is wearing a kilt un-American? No. Is wearing a kilt going to make people ask you about Scotland? Yes.

Cordially,

David