It's increasingly standard now in San Francisco - the last three reservations I have made have all required credit cards. Fortunately not yet in London.

I always do this via Open Table which I would hope has decent DSS. I would be amazed if any restaurant taking credit cards over the phone would meet their contractual PCI-DSS, unless they have the discipline to enter it into their POS, which I suspect that they don't. I'd love to see such places' ability to take cards withdrawn.