I found this mini-cube speaker at a store in town and tracked it down online. It requires no batteries, since you charge it using a USB-port, and it holds its charge fairly well. (I've yet to run it all the way out). Probably more suitable for overnight-weekend trips, but still, a super light-weight option for sound on your iPod. Lighter than the other speaker systems I've seen.
http://srv2.kitestailstoys.com/product.php?productid=23950&cat=0&page=1


