It would actually be mono, but that's irrelevant.
I've been thinking of an idea for a project to construct a battery holder for a powered computer speaker in order to have music on my rides. The speaker, battery pack, and iPod would be in my trunk bag:
My cousin made a battery holder for D-cells when he needed to power his iBook longer for a flight to Australia. here's the link: http://kc0wys.com/projects/electronics/05/index.html
Basically I would chop the (to speaker) DC power cord off of the wall-wart and connect it to the battery pack, thus this won't work with a speaker with an internal power supply.
The batteries would be D-cell NiMHs from Harbor Freight.
Anyone have any better ideas (or cautions, etc.) please let me know!