the main problem that i have read concerning wireless speakers is it still needs a source of power requiring you to run a power cord to you "conviently" close plugs that you thought ahead about. this basically defeats the purpose of wireless considering you still have to run cables just to get power. then theres the issue of signal quality that edster brings up. even bluetooth is still alittle too slow to carry the full bandwith in comparison to what is sent on conventional wires. i say its a fad, just go with regular speakers.
I have a set of Acoustic Research 900mhz wireless speakers that I purchased (I think I got them for about $199.00) to use outdoors. I have the sending unit inside and then would bring the AR's outside when we were on the deck. My kids take them out to the swingset/fort outside as well. The sound is boombox quality, nothing better.
They can use batteries or an adapter. With rechargable batteries they last about 2 hours or so, much longer with alkaline batteries.
Once I decided to try them as surrounds just to see how they worked. They are 10watts RMS (if I remember correctly) and it was hard to balance them in the system (requiring many up and down trips off the couch to change the volume). The sound quality was no where near that of my wired surrounds.
For bringing outdoors, they are wonderful. For surrounds, I would figure out how to run wire and use regular speakers. Perhaps others know of better sounding wireless speakers that function better in this role.