I was looking for a speaker the same about 3mos. ago and google lead me to this:
http://www.ohgizmo.com/2008/01/09/ih...peaker-system/

I found it for $55 by doing another search..many times it was out of stock.
It's on my bike here: http://www.flickr.com/photos/dclove/...in/photostream

The speaker is pretty rugged, I've dropped it a few times and no problem. I put my iPod in it and leave it on shuffle, and you can skip songs and adjust the unit volume from the remote that comes with it. It really is a cool gadget...I also carry it around on my shoulder while working in the garden and keep the remote in my pocket when I want to skip a song.
I've ridden with it playing steadily through a light rain, but in a down pour I'd put the lid on it.
The volume is plenty loud at the top notch, but it'll be drowned out when riding in city traffic.
It takes 4AA batteries and if using fully charged NiMH batteries, I've been able to get about 2full days (~10-12hrs) of riding out of it b4 the batteries die. I use a Brunton solar charger to charge my ipod and cell phone.

It also makes an excellent wedding gift for your cycling friends.

Lame plug...I know.
I also turn it off when I want the quietness of only my 2 wheels on the pavement and the environment surrounding me.