In many countries the Samsung Galaxy S phones are sold SIM locked to local telephone carriers. This prevents you from using the phone on a different carrier.
To unlock a phone, you need that phone’s particular unlock code. Each phone has its own unique unlock code. Most GSM phones are easy to unlock. Samsung Galaxy S phones in many cases are especially easy to unlock. This is because that unlock code is stored unencrypted in a file on that phone. So once you know where to look, you can see read the phone’s unlock code.
The developer community has made this even easier by writing apps to retreive the code for you. The easiest way is to go to the Android Market and search for an app for your Samsung Galaxy S phone.
In Canada the first Galaxy S variant was released on Aug 6th. I bought one right away, but could not lock it at first as the hackers had not figured out an unlock method. This changed on Aug 21, when they discovered the code was stored in the phone. A day later a script was available that went into the directory that contained the file, opened the file with the unlock code, and displays it to the user.
Here is the video how-to on unlocking your phone using the script from XDA-Developers.