Unfortunately, it's rather difficult to carry around a gaming system, a television, and a Guitar Hero guitar everywhere you go.  Luckily, there are alternatives for those of us who get that Guitar Hero urge at random times in random places (but mostly while out drinking at friends' houses/frat parties/dorms...):

The Air Guitar Hero : A t-shirt  that you can play "acoustically"

Pocket Guitar Hero: The Game Boy of Guitar Hero

YouTube: Where at least you can see other people playing Guitar Hero, which may be enough to hold you over until you're within reach of your beautiful plastic baby