The Xbox One Stereo Headset Adapter is now available today. This will allow your headset's from your 360 to now work on your Xbox One for chat (no longer requiring the use of kinect as a mic). But expect extremely high demand for these for a while. SO GET YOURS FAST!
Best buy still has them in pretty much every store for in store pick up from what I can tell for the MSRP of $24.99.
Gamestop has it as well (for a markup of a further $5, bringing it to $29.99...)
www.gamestop.com/xbox-one/accessories/xbox-one-stereo-headset-adapter/113825 (Copy and paste)
However they have it available for shipping while BB seem's to be sold out.
If you are looking to get it, place an order now, you can always cancel or return it but don't hesitate.
Both BB and Amazon are sold out for shipping, which is a sign of low availability across the board. So while MSRP at BB it is still a deal, as you can expect these to sell out for at least the first batch.