I was reading the directions on Knitty, and it said you could brew the yarn in a ziplock bag like sun-tea. So, I fixed up a bath for the mystery yarn (which started out a light beige) before I left for work, and this is what I came home to.
Pretty good for a first try, I think. :) I used two packets of Flav-R-Aid in Kiwi Watermelon, and one packet of cherry. The yarn is unknown fiber, unknown amount, unknown weight, but it took dye like crazy. The color is losing intensity a little as it dries, and it took better in some spots than others, but I'm still happy with it.