I've thought about this a lot myself.
I have a 2nd generic Xbox 360 wireless adapter and have broken it down to just the chip inside (which is still MUCH larger than it needs to be - tons of blank board space all around). It's small enough that it should slip right into the official Raspberry Pi case with maybe even some room to spare since the Pi is tall where the 4x USB ports are, with a lot of wasted space behind it in the case.
I'm currently researching if I can hard-wire the 4x wires of the USB plug to the Pi's GPIO wires (which I don't think is possible), or how to most easily connect the wireless adapter inside the Pi's case so it looks nice and neat from the outside at least (if anybody has links to any tutorials I'm all ears!).
The problem I haven't figured out yet is how to select which player is which. I commonly have an issue where the Xbox wireless adapter steals player 1 even when it's plugged into the traditional Player 3 or Player 4 USB slot (this is when I plug in my 6-button Genesis controller for example, for some Streets of Rage action ;) and the only way to get that Genesis controller to be player 1 is to unplug the Xbox wireless adapter...which won't be possible if I hard wire it (unless I put in a switch or something). Might just snake a very short wire out of the case to a regular USB plug if it would look OK.
