The XCM XFPS 360 was announced a few weeks ago it now available for pre-order, this will enable you to add a standard keyboard and mouse to your Xbox 360. As you can see from the picture is has multiple ports for Xbox and PS2 controllers.  In addition, it will enable auto fire, and  it might be able to use on Halo 3. Not much more information is available from the website other than it's still in prototype and will be available soon.

UPDATE: Here is the manufacturer's website, the last site posted was a prototype development site.