> As for inserting keystrokes, that will be obvious if it enumerates as a keyboard.

This is true, but this also doesn't need to happen at insertion time. An HID keyboard can show up, say, 3 hours after you plug it in.

I miss grsecurity's patch set so much. It had an option to defeat this (deny all USB device enumeration post-boot, i.e. after the kernel executes init).