The closest thing I can think of hits 3 of your 4 criteria is the ThinkOutside Stowaway Bluetooth Keyboard. The only thing it doesn't do is tether with a USB cable.
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(http://l.mashby.com/2vLr+)
ThinkOutside originally created a folding keyboard for the Palm Pilot and then had different versions for different connectors for other PDAs. It was a PITA to upgrade and then have to buy a whole new keyboard, but I did every time there was a change. Eventually, Bluetooth became a thing and they migrated to doing a single keyboard. It's chromed plastic and folds into a nice package. There is also a neoprene case that zips around the keyboard when it's closed.
After I switched to the iPhone in 2007, I tracked one down on eBay and I am happy to report that it works. However, I do have to re-pair the keyboard every time I use it and I don't know why, but when I need it, I'm glad that I have it in my bag. I paid $100 bucks for the one I bought 7 years ago, but there's one for sale on eBay for $78.26 (http://www.ebay.com/itm/NEW-Stowaway-Sierra-Bluetooth-Keyboard-Targus-Think-Outside-PORTABLE-/281310650177?pt=US_Tablet_eReader_Cases_Covers_Keyboard_Folios&hash=item417f6bdf41)
ThinkOutside also made a smaller version (http://www.pocketgoddess.com/2006/07/think-outside-stowaway-universal-bluetooth-keyboard-and-stowaway-bluetooth-travel-mouse/), but I preferred the original. I really liked the ThinkOutside company and I was sorry to see them get acquired by iGO and then later fade away. Their foldable keyboard was very unique for it's time and I used the heck out of them back in the day. :'(
Hope that helps!
This was the model I had, while it was as nice as a laptop keyboard could be, it is far from the feeling of a mechanical one.