Thanks.

The advantages of having it connected through USB-HOST are big:
-you can play with the system in hand (unlike BconTrollers)
-you can charge your phone if internal battery is installed
-can make gaming more portable (Sixaxis makes my butt look big
-can set the gaming layout as you wish (some have big hands u know
-might be faster than BT controller (depends mostly on drivers)
-it can have other functions (eg an extra SD slot, WiMax antenna etc etc)
-cost (can be cheaper)

So what's necessary (afaiu) for this is:
-firstly get USB-HOST to work
-fix bugs so commands are quick (ie no lag)
-develop a driver/drivers for such an accessory
-then actually make the accessory

Just to illustrate again: