I like it. Seems like it might be useful for mice and keyboards as well.

Have a few ports on it: This device can only be a mouse -> USB This device can only be a keyboard -> USB

Then it filters everything coming in to ensure that it matches the desired type of activity.

For USB drives, I'm tempted to say it should read the USB drive once, and copy all information to internal storage in order to prevent data being sent to the usb via timed or coordinated reads. This would allow a truly read only thumbdrive.