In the wake of the news about the Target breach, here's a story telling what most likely happened at a Midwest grocery chain this summer. Because everyone who shopped at Schnuck's was affected, pretty much everyone in large parts of several states were inconvenienced and issued new cards.
As I noted in a comment to Gawker, my bank sent me a letter and a new card. A friend of mine was out at dinner when their card was declined. After settling the bill another way, she called her bank and learned of the breach. They had preemptively canceled every card which had been compromised. While another person I know, their bank cut off the debit portion of their Visa check and would only let them process as credit, so there would be no cashback involved.
For anyone interested in more information about this summer's Schnuck's breach, here's a quick and dirty search of the St Louis Post-Dispatch.