Adafruit Industries just announced their next kit: a SIM card reader. Using the kit, you can read or write any SIM card. You could use this for fun things like recovering deleted contacts and SMS messages. The kit looks like a very straight forward design (based on Dejan’s work); the only chip is a hex inverter and the board is powered by a regulated 9V battery.
With all through-hole components, it should be easy to assemble. You can talk to it using the board mounted serial port or connect to the extra pin header using an FTDI USB cable just like the Boarduino. The FTDI option is bus powered, so you won’t need the battery. ladyada has collected some resources in case you want to learn more about smart cards.