We just posted about an awesome hack named Subscriber Artificial Module (SAM) which allows iPhone users to officially activate iPhone 3G, 3GS & 4 without Stock SIM. However you must have jailbroken iPhone to install SIM and then revert back to Lockdowned mode by running Stockify on SSH console if you had hactivated your iPhone with Redsn0w or PwnageTool.
Ssteps are difficult to perform for navice users. Well here is good news, to support SAM iPhone dev team has updated their Redsn0w 0.9.6 4.2.1 jailbreak tool to Redsn0w 0.9.6b5 which has an option to diavitvate iPhone to allow you to use SAM to do official activation.
Download Redsn0w 0.9.6b6
(As it happens due to overload the download links may become inaccessible for some time)