questionsbluetooth headset with aux 3.5mm jack output…

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It does exist:
http://deals.woot.com/deals/details/840038ae-4be9-4d03-9ad2-c11e78b48cd0/motorola-s705-soundpilot-stereo-bluetooth-headset

It has some extra features you do not need like a built in radio, but this is essentially what you are looking for, a Bluetooth headset where the headphones are removable and since the headphones are removable it has a 3.5mm headset jack.

Edit: BTW, I really love questions on whether a type of product exists. It is always a good question, which leads to the hunt for a solution to a problem.

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@gestaltx: You are welcome. Glad I could help.