There was a question on FSB on how shielding works... 

What happens is radio waves induce eddy currents in the metal. When grounded these happily flow away, but if there is any impedance to ground that can allow a potential difference, you get reradiation. The dimensions of the box will define the resonant frequency.

Here's a practical example...

http://berkeley.intel-research.net/arahimi/helmet/

For a slightly more theoretical approach take a look at

http://www.analog.com/static/imported-files/application_notes/41727248AN_347.pdf