There is a simple problem, if someone decides to play with a keyboard, then he (or she) would probably react faster. Hacking games to make them believe that during a multiplayer game you're playing with a helmet can't be that hard. That would give a cheater an incredible advantage.
Yes, i saw that in muthbusters. But I prefeer to use the CD as a frezbee. After hitting a wall for the third time, i guarantee the cd is succesfuly destroyed.
I think that the concept is very nice but.... what about other guns? I mean, for example, HL2 had the innovative gravity gun (or zero point energy gun, it is the same) but the game was much more than just one good idea. Maybe I'm wrong, but in my opinion, Portal will be quite boring after the first hours of gaming.