For those who don't know, the key elements of RFID technology are the chip and the reader. The chip or "tag" is a small electronic device containing a radio antenna and a small amount of circuitry, which can store some data (typically a unique ID number) and perform a tiny amount of computation. The reader is a combination of directional radio transmitter and antenna that fires radio waves and listens for responses from RFID tags. Passive RFID tags don't need a battery; they get power via induction in their antennae from the reader's radio transmission. RFID tags are durable and cheap (last I heard, less than 50 cents each), and, like all electronics, getting cheaper and more powerful all the time. Most consumers have probably seen RFID tags used as anti-theft tags, though they have many other uses.
RFID tags can be read without physical contact, although they often require that the tag be held in the proper orientation relative to the reader. The range at which they can be read by current technology varies from centimeters to several meters. You can increase read range by using tags with bigger antennas, and by increasing the reader's power output; but the physicists tell us that, for passive tags, the strength of the tag's reply drops as the fourth power of the distance between reader and tag, so it seems unlikely that you'll get much beyond tens of meters in the foreseeable future.
So, anyway, as I was saying, if many companies follow JP Morgan Chase's lead, and decide to embed RFID tags in their cards --- and many of them will --- then your wallet will be packed with RFID tags containing all kinds of interesting information about you. Anybody with a cleverly located reader (one Politech commenter suggests planting one in the seat of a mall bench) will be able to snag the entire contents of people's wallets, without the victims' ever noticing.
Fortunately, RFID readers can be foiled (ha, ha) by all the usual countermeasures that work against radio signals. If you line your wallet with aluminum foil, it will become impossible to read your cards until you take them out. Of course, that countermeasure has the downside that you will have become the kind of person who lines their wallet with aluminum foil.