Corsaire has published a rather lengthy paper on attacking gift card systems. While this is a little off topic it's a good read.

"This paper is based on research conducted on a large number of UK gift cards. It has been created to complement the presentation “Stored Value Gift Cards: Magstripes Revisited”, which was delivered at the EUSecWest security conference in London in May 2009. It concentrates on magnetic stripe (magstripe) gift card attack techniques and also provides a series of guidelines and tips for developers and systems architects who are involved in the process of implementing their own gift card technology."

Read more: http://research.corsaire.com/whitepapers/091021-attacking-magstripe-gift-cards.pdf