Can you explain why it would be offensive? I went to a Native American presentation a couple months back and that poster didn't seem too far off from what they showed us as cultural clothing (the feathers definitely were in this presentation - not necessarily the beads on the shirt - but mocassins for sure they talked about). Is it just the shirt that you don't think is proper for that culture? Or is it dressing like a Native American at all that you think might be offensive? I don't get it.
Sure...
Traditional outfits are now a sign of honor (whereas a lot of them were actually every day dress "back in the day"). Dancers wear them, people of great importance, etc. Feathers must be earned (and they certainly aren't of the neon variety). I, myself, only have three, aside from the feathers in my fan. Beadworkers spend countless hours searching for the perfect beads, and then days, weeks and even months beading their pieces. These are added to the attire in the form on medallions and other pieces. Mocs are given at important stages in a life, and they aren't even worn in some tribes (one of my tribes wears them, the other wears leggings). This is another facet, completely, of the portrayal of Indians as one people. There are 39 federally recognized tribes in Oklahoma alone, all with unique dress, culture and traditions. Fringe is not used often. It is used on dance shawls (and they not cut into the fabric, it is added, strand my strand) and on buckskin dresses, which are a huge deal. Here is a pic of one of my tribe's traditional buckskin outfits:
My sister is on the far left.
All of these girls have been the tribal princess or have been honored in some other way. You must have done something honorable to wear these symbols of the past. Indians aren't a character of the past, but a living, breathing culture that is still very important in the lives of many Natives. Also, I am only speaking on the knowledge I have of the tribes in Oklahoma and surrounding areas.
Believe me, the picture posted bears little actual resemblence to the attire it is meant to represent.
If you'd like more info, PM me, as I'd hate to hijack.

Sorry, OP, for this OT post!!
