i've played around with some, and the technology is not quite there, at lease in a mobile sense. ibon is correct, if you want a good set of wireless headphones, it requires a dedicated base station, that's usually pretty bulky.
i know that there are some professional systems out there that have a very good quality, but you pay through the nose.
there are bluetooth headsets for cell phones, but i haven't tried them, and i would think that the quality is still not aas good as a wired set.
i think with the current wireless movement we are seeing in computers and technology in general, we might see something of good quality, within a consumer price range in the next couple of years, but nothing quite yet.
<grain of salt>
until then, get an itrip, and headphones with a fm radio built-in, and you're set!!!!!
</grain of salt>