I have given the same advise about the GM layer for those GMs looking to hide their minions before unleashing horrible death on their unsuspecting players Muuhhaaaaaahahahahaah ! er excuse me that slipped out :-)
On another note, what I would also like to see (and what I think some people are also referring to) is how can a player make their own token invisible and still maintain control?
I mean there can be some work around such as the GM taking control of the token and placing it in the GM layer, but the player may feel as he/she is being left out. You can also put a small (5 ft) aura around the PC and state that that means it is invisible and the player still retains control, but what I think would be really cool is if the player token can be given a semi transparency effect so that it looks like it is hidden (similar to the view of the tokens on the GM Layer).
Just a thought.