I've heard of old worn out jeans becoming attic insulation. I just never knew where to donate them. Well, thanks to that recycle shoe thread, I went hunting and found 1 site. You have to mail them at your expense, but UPS ground or Fed Ex ground or even the USPS would be cheap. Since they are worn out, you won't want to insure them.

http://www.fairindigo.com/recycle/