First off, you're literally the first person I've seen that would make use of a feature that I believe is standard in ICS.
When you tap a text entry box that pulls up the keyboard, you should see a keyboard icon in the notification bar. Pull the notification bar down and tap where it says "Select input method". That pops up a box for you to change inputs. As far as I know, you couldn't have turned that setting off. There was a lively thread recently on here debating if that was a feature or a bug, and if it could be disabled. It was established the shortcut was a feature that can't easily be deactivated.
Btw, I'm curious why you want this ability. No one in the other thread really had a good reason, and I've wondered what a good one would be.