I always understood that when you ran your debit card through signature based since it was going through the credit system you were covered. I recall banks advertising this and I have had to use it twice, one time my card was copied by a gas station the other was when when Yahoo cached it in their store processing system, both times the bank did treat it like a credit card and refunded my $.
I've been through both sides of the Visa agreement a few times, you know when you get those e-mails telling you to write 'see id' or somehting else silly on the back of your card to 'protect' you, respnding with the part of the visa agreement forbidding that helps.