RE: Jolyon Ralph
That's exactly how it is done.
The credit value is stored both on the card and on a central database because it's not possible for real time authentication via the central server for all transactions. So instead the transactions are logged, and the readers take the cards balance at face value, and the money is taken off the card. Then there is a nightly audit ran through where the transactions are processed on the central server, taking the balance off there and comparing the cards stated balance with what the actual balance should have been.
This is why they state that the most you can gain from this is one days travel, the nightly audits will flag up any discrepancies in the value the card states and the value in the central database, and will therefore flag the card as being cloned.