Oddly enough, I was thinking of a service like this when my credit card was declined in London. The problem of course is someone robbing you + your phone.
The company recently formed a partnership with ValidSoft and is going to start using information about the location of customers’ mobile phones to prevent credit card fraud.
Visa will be able to establish whether your mobile phone is in the same place as the merchant or ATM where your card is being used. In most cases, the two devices are in close proximity, which allows Visa to surmise that it is probably you using the card (even if you are outside of your usual shopping patterns). However, if the two devices are not in the same place, the system may send up an alert.