With new contactless credit cards you can make small purchases without resorting to the Chip-and-PIN transaction that is most common. Instead, you just 'touch' your card on the reader and away you go. The problem with this is that you cannot turn your card off. I can bring the reader to you; I just need proximity. These readers are small and pocketable, and I can read your card without you taking it out of your pocket. The more high-powered my reader, the further away from you I can be to read your card. Initially, the cards gave out the name on the card, the card number and the expiration date. After people showed that it was easy to skim this information off the card, most have removed the cardholder's name from this list. They have also introduced transaction IDs to help protect the cards from being cloned. However, as the introduction of RFID identity cards increases, we will be giving the cardholder's name out again on those.
It has been argued that it is more productive to attack the databases of card details, and still far too easy, so people won't bother trying to read the cards in your wallet. However, I can still obtain a legitimate payment reader and just read your details off and collect your money directly, even if I can't clone the card. In the UK, these contactless transactions are for small amounts of money (£5-£10 typically), but I can collect that from you without your knowledge in a variety of ways. I can use a small pocket device to take a payment from your card directly, but this has to be one at a time and, in some countries, only for small amounts of money. (See the video below for a mobile phone that can process contactless credit card transactions.)
Maybe this isn't worth it to anyone but a petty criminal, but it is relatively easy and cheap. Another way would be to go to a crowded area (public celebrations or gatherings of tens or hundreds of thousands of people for example) and use a high-powered reader to read lots of cards at once. If I can steal £10 from 100,000 people, that's £1m in an afternoon! Somewhere in the region of half a million people gather in Trafalgar Square, and environ, for the New Year celebrations and similar numbers for the Chinese New Year celebrations a little while later.
RFID readers are all over the place and we don't pay them any mind. Shops use RFID readers to catch shoplifters at the entrances. Do you categorically know that they aren't reading your cards instead of catching shoplifters? Would you notice an extra £10 transaction in a shop that you did buy something in? All of our buses and tube stations in London use RFID readers for ticketing. A vast number of commuters will have their Oyster card in their wallet or purse along with their other cards. They don't remove the Oyster card to touch it, they just touch the whole wallet. I could read the other card information at the same time. In fact, do we know that these details aren't logged and just ignored? If they are logged, then maybe we could attack Transport for London's computer system and extract people's credit card details.
Maybe people won't do this with credit cards. What about large banks or other companies that use RFID door entry cards and ID cards? I could read their ID and possibly gain access to their building. Will the ID give me a username to work with? If I can gain physical access to a bank building then I have a huge array of attack vectors at my disposal; it is critical to keep the hackers out.
Even if a hacker can't clone these types of cards, they can still collect information about people and about companies. It is perfectly possible to identify all the employees of a company using RFID cards for door entry, as you can have a high-powered reader near their entrance or at public transport stations. This now gives a social engineer a target and some information to use. Maybe we should all be holding our wallets over the top of shop gates and away from other readers or buy one of the shielded wallets from people like ID Stronghold or Herrington.