Grundig offers a nicely designed wireless outdoor speaker dubbed PSW 5000.

The Grundig PSW 5000 features two 2-way loudspeakers and 20 W audio power. The speakers are mounted in such a way that 360 degree surround sound comes from this wireless speaker rocket. The casing is splash proof and robust, making it usable for the outdoors. Besides receiving audio from the wireless unit a direct line-in is also available to connect your mp3 player.
More details on the Grundig site. Available in Germany at Technik Direkt for 126 Euro. I could not find a store in the US yet that carries the PSW 5000.