In all actuality, iris scanning isn't so futuristic after all. These devices not only exist, they're being tried out at various locations around the world, although they aren't quite in active use yet.
The idea behind a iris scanner is just what sci-fi told us it would be. Everyone's iris is unique, and easily distinguished by modern computers from the iris of any other person on the planet. In fact, irises are more easily distinguished than either fingerprints or signatures, both of which are used today as evidence of identity. Push your finger down too hard and you can throw off a fingerprint reading device. And it's by no means impossible to imitate another person's signature!
But iris scans are almost impossible to fake. As you walk toward the bank teller a camera zooms in on your torso, finds your head, then finds your eye. It snaps a picture and then breaks down the iris markings into dark areas and light areas. This data is compared with a scan you put into the computer's memory when you opened your account. Bing! You don't even need to swipe a card.
Some systems even check for the tell-tale contraction and dilation of the iris, to prove that the eye being scanned is indeed alive.