When buying a 3G Apple iPhone in most markets you are automatically tied to a network such as At&T in the US and O2 in the UK. This means you won??t be able to use any other operators sim card in your iPhone.
Deal Extreme have just launched the i-SmartPhone Operator Network SIM Card Unlock Attachment for Apple iPhone 3G that frees your iPhone from the network lock allowing you to use any other operators sim card in your iPhone that costs just $16.99 including shipping worldwide.
More info at Deal Extreme.