How to create a wirless audio headphone
I was looking into exactly the same project and I rules out Bluetooth and Wifi. Bluetooth has some serious interference issues, especially in places where wifi networks already exists. It will not be very reliable and sound quality will suffer. 

The best solution I have found so far is Kleer Wireless technology which is distributed by SMSC. It's the technology used by Sennheiser for their high end wireless headsets. Like you, I would like to use my own headsets, I have nice Bose QC15 and they would work much better than any pre-built headsets.

Here is a link to their chip: http://www.smsc.com/index.php?tid=346

I did contact them and they have sent me a bunch of stuff but my knowledge in electronic is limited and I have to say I'm a bit stuck..

[Update]
Since then I have actually made progress. I bought a used pair of Sennheiser RS160 and took the headset appart. Then I unsoldered the battery holder (which are part of the headset, and un-soldered the speakers and put and female 3.5mm Jack instead. I bought a standard battery holder for 2 AAA and here you go. Slapped all this in a box and you got your self a device on which you can plug your nice headset. Works like a charm.

hope this helps you,
cheers
Antony