At least one bank is informing their customers.
"Your Visa account number was in that group of accounts. We have already established a new account number to replace your old account. Please watch for your new cards in the mail in approximately 10 business days. We will not block your existing account until we believe you have had time to receive the new cards. While we believe that your account is secure we are taking this step to insure that your account will not be used for fraudulent activity in the future."
The letter goes on to remind me that I am not liable for fraudulent transactions and that I should review my statements and make any disputes in writing.
They got my street name wrong on this letter; my bill (which arrived in the same batch of mail) has it correctly. This suggests that someone is putting in a lot of keyboard time.