See: http://ubuntuforums.org/archive/index.php/t-231700.html

KVM switchers read the ScrLk LED, switching computers when the see the LED toggle. Normally, you toggle it by pressing Scroll Lock twice. Ubuntu doesn't accept ScrlLock, and doesn't turn the LED on. Not finding a way to enable it, I opted to use the suggestion in the linked article, and created a KVM switching script.

The script here creates a new command, switchkvm.

echo "xset led on; sleep .25; xset led off" > switchkvm
chmod a+x switchkvm

I put an icon in my toolbar so it's one click away. Attached in an ugly icon for it.