A quick perusal of the link here:

http://www.kiltiebears.com/tartanbonbright.html

Reveals a number of USA tartans...here's what I noticed, just scanning the list quickly.

Confederate States commemorative (for you southern celt rebels!)

Georgia District...USA Bicentennial...US Navy...US Marine Corps...Texas Blue Bonnets...South Carolina District....NOrth Carolina District....Rotary Internationl (???)...

Though I don't know if these are registered tartans or not. Very nice.