well i looked a while but i didnt really find anything useful. One significant problem with wireless headphones is interference from other devices or simply because there are things obstructing the signal. (such as walls) Also, 2.4ghz wireless headphones operate about the same frequency as 802.11b wireless networking, and it can also interfere with the signal.
What are you going to use them for? do you just hate wires, or do you find yourself wandering the house wanting to listen to music at odd hours of the night and not wanting to wake anyone else?
well thats all i have on the subject right now.