Wow, I just googled it and am blown away by the existence of this product.
My "case" is also a sock, but it's one that formerly belonged to my 3-year-old son. The matching sock is missing, so my total cost = $0.
If I want other colors, I just wait until my son loses another sock. I have my eye on Lightning McQueen.
For extra protection in my briefcase, I sometimes slide the iPod+sock into an old leather Handspring Visor sleeve. It's very professional-looking, and really impresses all the other top executives when we have a board meeting.