It sounds like science fiction, but a new idea could lead to a form of shielding that would make an object effectively invisible. Ulf Leonhardt of the University of St Andrews in Scotland and John Pendry of Imperial College London and colleagues argue that suitable "metamaterials" could be constructed, at least in the microwave region of the spectrum, that could bend electromagnetic radiation in such a way as to deflect it around an object and then send it on its way as if there had been nothing there.
Normal materials with the required properties are unlikely to be found, so complexes of metal rings and wires would be used to generate the appropriate exotic behaviour. Of course even if this idea works for microwaves, it may not be possible to engineer the right sort of material for the optical region of the spectrum, or for a range of frequencies wide enough to make something that Harry Potter could use.
Further reading
U Leonhardt 2006 Science 312 1777.
J B Pendry, D Schurig and D R Smith 2006 Science 312 1780.