If you are not using an official SIM (say, the one from AT&T), you should be very familiar with the below message shown in iTunes when you connect the iPhone to your computer:
The SIM card inserted in this iPhone does not appear to be supported.
Apparently, you can use PwnageTool or Redsn0w to activate the iPhone (i.e. what so called hactivation). But no all iPhone users are comfortable with these jailbreak tools.
Sherif Hasim, the guy who found the exploit for unlocking iOS 4, has figured out an easy way to activate most models of iPhones including iPhone 3G/3GS, as well as, iPhone 4, without an official SIM. The trick is to make use of a Phonebook SIM card. As you’ll see in the video, you can simply insert a phonebook SIM into the iPhone. And, that’s it. The iPhone is automatically activated.