XIM360 to Bring Keyboard / Mouse to Xbox360
The XIM2 is not exactly cheap, at $179.95. But it does come pre-built (check out this .PDF to see what you would have to do to install it yourself) and includes an Xbox 360 wired controller, so that's included in the price.
Image Source: XIM360
The diagram above seems to indicate that unlike the PS3, which can use a keyboard and mouse only if the game is coded for it, this should work with any Xbox 360 game. And let's be honest, nothing beats a keyboard / mouse combo for a few rounds in an FPS. Still, $179.95 is pretty pricey. According to the site, the XIM2 will be in stock on Jan. 19th.
Would you readers be willing to spend that much?