Software changes needed to make an Android phone capable of USB jacking:

• you need to run the USB port in accessory mode
• android-bluetooth-usb-hid
• Angelos Stavrou, Zhaohui Wang, "Exploiting Smart-Phone USB Connectivity For Fun And Profit", Black Hat DC 2011 [slides] [paper] [layperson summary]
  "we developed a special USB gadget driver in addition to existing USB composite interface on the Android Linux kernel using the USB Gadget API for Linux. The UGAL framework helped us implement a simple USB Human Interface Driver (HID) functionality (i.e. device driver) and the glue code between the various kernel APIs. Using the code provided in: "drivers/usb/gadget/composite.c", we created our own gadget driver as an additional composite USB interface. This driver simulates a USB keyboard device. We can also simulate a USB mouse device sending pre-programmed input command to the desktop system."
• efforts to get the Gadget API working in the kernel