Although currently somewhat expensive to implement, that's due to their recent development and marginal-but-increasing spread in the market. In the future, in theory, it could be used to enable instant supermarket check-outs: walk by the checkout machine and it registers all the items you're taking out of the store and gives you a way to pay for all of it - or, if your RFID loyalty card is similarly tagged, it can automatically deduct the cost from your account.
Months ago, when I was complaining about how easily books are misshelved, C. commented that RFID tags would be ideal for tagging books for easy tracking on shelves. The problem with reshelving books is that collections can be enormous, call numbers long and complex, and the reshelvers are underpaid human beings who do their best, but slip up now and again. When the first three lines of a call number go on for three aisles in the main library, a book misshelves - be it one or ten shelves or cases away - can prove a book permanently lost, for it is no longer findable.
The Vatican Library had exactly that problem. It had to close for a month every year to do inventory, to find lost books, to find what books it still had and to make sure they were all in the right places. The big news at the moment is that they're implementing RFID tags throughout their collection! The Vatican Library now estimates that it'll take half a day to do a full inventory instead of a month. The handheld monitors will register the presence of the tagged books on the shelves. Intelligently, these tags will also have text on them for easy confirmation that the right tag is being installed in the right book in the first place. The handheld monitors make it easy to find a mishelved book, especially if it was misshelved near where it was meant to be: all it takes is a walk along the shelves with the monitor in hand. (More on the complications of running the Vatican Library here.)
I wish more libraries would go this route: it solves one of my biggest frustrations with the libraries I'm working with at the moment.
On the other hand, I hope that passports and other forms of personal identification do not go this way... just think, any random person on the street, criminal or security alike, could be reading the passing tags to find out who you are, where you're from, and anything else about you on those tags... but that's another subject.