For those who don't want to go to all the trouble of running wires out to a fancy rock speaker in the garden, Creative has a solution: the $199 Sound Blaster Wireless Rock Speaker.
Creative says the Wireless Rock Speaker is a "high-quality stereo speaker encased in a granite-rock-like exterior," and is designed to "withstand moderate weather conditions so it doesn't have to be brought in every time it rains." In other words, the speaker isn't entirely weather-proof, but it seems to be water-resistant.
Alas, the Wireless Rock is not a Bluetooth speaker but works with the $100 Sound Blaster Wireless USB audio system, which is available separately (bummer). Once you plug the USB accessory into your computer, you can then stream music wirelessly from iTunes, Pandora, or other music apps (the speaker must be within 100 feet).
Creative's Speaker Rock gets its juice from a removable rechargeable battery that delivers up to 6 hours of playback. Creative says the battery recharges through a separate powered charging cradle (included), and the battery plugs into the back of the speaker, "so the speaker does not have to be moved in order to be charged."
In all, this sounds like an interesting and affordable solution for an outdoor wireless speaker, though it would have been nice if you had the option of plugging it into an outlet and leaving it outside for good.