Not just the iPhone version, but all the way up to 720p if it's available (I believe it's the video the flash player serves you when you append &fmt=22 to the URL, if that still works).

Mastercard definately do, although I can't find the PDF with their merchant guidelines in that I used to refer to. I've worked at a UK based retailer in the past, serving a customer with 'See ID' on the back of her Mastercard. She looked at me incredulously when I refused to accept her ID as proof of signature on the basis that I have no idea what a valid State of Connecticut drivers license looks like (and as parent said, it invalidates the card). She told me that "a policeman told me to do it for security".

Also, when it comes to checking for signatures on Chip & Pin based cards, generally no signature just means a lazy customer. The words "VOIDVOIDVOIDVOID" where the signature strip should be (which is what happens when you try and remove it) is a much more obvious sign that something's odd.

Another customer told me he refused to sign his cards "because then a thief could then learn my signature and use my credit card with it", "But surely leaving the space blank just means that the card thief would just write their signature in the space and save themselves the effort", "...Could I borrow a pen?".