I started out looking for a way of backing up my mobile phone SIM card, mainly because I have two mobile phones and I wanted to save wear and tear on the card by repeated removing from one phone and inserting into the other (one is large and bulky but includes better PDA functions for mobile working, whereas the other is small and slim and better for slipping into a pocket when going out socially or when a lightweight phone is required).
It seems that in order to clone a SIM card you need to know the IMSI and KI values stored on your original card. Because UK network providers don't like you cloning cards, they've changed the algorithms used to generate these codes, so at present (November 2008) there is no known software to crack the KI value. However, the following links may provide more information.
See also my cryptography and privacy pages.