There are active USB switches (as well as active PS/2 KVM's) that provide a "ghost" device on each port so the machine's never see the disconnect. For a lot of machines it is the only way to go, as they are very picky (especially with PS/2).

Well, wicked pissah! I finally took a moment to hook a Logitech Force joystick up to 1shared port on this IOGear KVM switch and my HP 6350 USB scanner to the 2nd shared port. Both behave as you suggest -- they are detected when "new" to one of the machines, but they don't disappear when switching to another machine -- no problems with them appearing to be unplugged.

USB side note. I couldn't resist this little thing. I have 3 PCs' sound output mixed through my main desktop and I wanted a way to adjust the volume/mute without messing with the keybord and this little aluminum jobby provided the answer.