Wired has and excellent article tilted The RFID Hacking Underground and it is a really interesting read on RFID security problems.
I checked out the RFID lab at work recently and found that there is far more about RFID than I thought I knew. For a start there are several passive chips, most being the short range ones (several cm range). The new UHF chips work at a range of several metres, but are much larger in size.
RFID is booming at the moment because of the UHF chips long range, and the fact that the US and Europe have agreed on a common frequency to operate them on, so you can use one chip both sides of the Atlantic.
Work has a cool demo of a goods truck being loaded, and the truck actually tracks what is on/off the back of it. It will tell you when you have brought the wrong container on and should really speed up shipping of goods eventually (If you have seen the IBM goods tracking ad where the boxes are telling the driver he is on the wrong road, then you get the idea)