I just bought a pair of the Acoustic Research AW871 wireless speakers and am pretty impressed with them so far. It takes them a minute to find the signal, but once they do they sound good. I think they are 10 watts each which is plenty loud for my deck.
I bought them off of ebay for a total of $62 with shipping and all for an open box set. At best buy they are $199. So I feel I got a real good deal.
One thing I like about them is the fact they can run off of the power pack supplied and/or you can put 8 "C" batteries in them and you don't have to plug them in, they are completely wireless and mobile. If you put rechargeable batteries in them, they will recharge automatically when they are plugged in.
I have mine running off of my Dish Network box with the Sirius music channels for a good variety of music, but you can plug the transmitter into anything with an 1/8" jack and it comes with a RCA adapter plug.
I will say it takes some time to get the level set correctly on the transmitter so it won't distort, the speakers are a little on the bass heavy side.
I have a small sound board I use with my electronic drums, so I ran the DN signal through it so I could EQ some of the bass out and now have a nice clean signal.
It also says that the transmitter will do 300 feet. It's probably more like 100-150 feet, but I am going through 3 walls too.
I can't offer any input on wireless speakers but I tackled this project a short while back at my pool. I bought the KLH rock speakers mentioned by Sean in the thread below. I was just looking for some decent sounding speakers in the pool area and the price was right on these so I decided to give them a try. I couldn't be more pleased with the sound delivered by these speakers. I'm pushing them with an old Kenwood tuner and they sound great. Since it's started to warm up lately we've had several pool parties and people have consistently commented on how good the rock speakers sound. It was a bit of a project running the wire but I love doing that type of stuff anyway so it wasn't an issue. Just another reason to be hanging out around the pool.