I have a set of RadioShack wireless. They run at 900mHz and have static issues. You can run them with power (defeating the 'wireless' aspect even though you can move them around your house and plug them in).
You can use batteries (I recommend rechargables), and they work great in that way. But they do have static on occasion and are not 'glitch free.' The sound is more like an FM radio, so for hi-fi sound you shouldn't even think about these.
There are 2.4gHz speakers out there, but these would likely cause your wireless airport or network to go south (same for the wireless phones that are in that range). I wouldn't recommend those unless you are sure you don't want to use airport or other wireless in the future.
But, with those disclaimers, i've thoroughly enjoyed having my CD collection ripped to MP3s and having them available anywhere in the house and even outside. I bought a headphones set that can tune the same frequency and they are nicer than the speakers (in my opinion). In any case, be sure and get rechargable batteries!