I too am interested in this area. I experimented with buying three wireless speakers getting their signal from the computer so I can listen to ITunes or streaming. They were on clearance at Amazon for like $18 each so didn't have much to lose financially but alot knowledge-wise. I think they operate at 600 mhz frequency, but the signal garbles when someone walks by a speaker so overall I like the concept but vote NO on this application. I'm now leaning toward hard-wiring to the deck coming off the stereo reciever, along with a wireless connection from the computer to the stereo receiver. There are now some independent remote speakers available for other rooms which might be good as long as the tech is improved over what I tried.

You might ask the question on AVSForum.com, I used that site to answer questions about building my home theater. From what I understand, without controlling the space of a room (impossible outside) getting good quality sound isn't going to happen. So you want to make sure the speakers can handle enough power to be heard and cover a wide frequency range.

The speakers are usually good for back-round music and not much else, this is not a high fidelity game.

If they get loud enough it can annoy the neighbors.

The biggest adjustment the speakers have is in speaker cone materials.
Plastics are very common to withstand humidity and moisture.

Look into marine speakers made for boats.

Keep what brickeyee said about annoying the neighbors in mind.

Thanks for everyone's input. I was curious about the wireless systems that seem to be all the rave now. We are building in the middle of a 400 acre farm, so our neighbors will only be the wildlife around. I just want to be able to enjoy some Itunes while I'm out on the porch if I chose. I'll check out the site bdpeck recommended. Thanks everyone!