I've been doing some further research on this topic, and firstly, I'd like to say "I was wrong" (dammit - happens occasionally) and the Rewards syndrome has been catching people out as recently as Feb 2008.
But on digging further, here is what I believe is happening:
When ordering your "free" business cards, you spend a chunk of time designing your card, reviewing it, saving it, and revising it. The interface for doing this is quite good by the way.
Then when you are satisfied and want to "order", you go through quite a tortuous process when you are asked to confirm your order sign for the design and enter you credit card details for postage - fine so far.
But to complete the order they take you through a further 6 or 7 pages during which time you are offered dozens of add-ons like an extra 250/500 cards, or fridge magnet, or card holder or notelets or stick-it notes and so on - all beautifully illustrated with your designs already present. Very tempting and quite annoying.
THEN...... towards the end of all this, when you're quite tetchy and fed-up with all the extraneous marketing bumph, you are also offered a £10 cash back on a future order. My memory goes a bit hazy here, but from what I've read, this is probably one of those options you need to explicitly opt out of. Its also not very obvious that in accepting this cash back offer, you are subscribing to their rewards program,
which further implies a monthly charge (my small print for emphasis !!!), although obvious enough that many of us have not succumbed.
I'm not taking sides on this issue and suggesting that anyone who falls for the "scam" deserves it. I'm simply trying to clarify that its nothing to do with stealing your credit card details. You took your free cards, you got caught up in some questionably over-aggressive marketing and on reflection, I understand the annoyance.
To those who have yet to use Vista Print. You have now been warned, but you also have a number of recommendations.
edit: Just found
this link on VistaPrint website. Seems to explain what I have been rambling on about.