Ok 1st the usual discalimer: I have not personally try this, so if it doesn't work don't come yelling at me. I found this on net on a reputed mobile site and had some good reviews, so all of u might wana give a go.

Here is how:
1.) Download the following file to your Mac. iPhoneTool.zip

2.) Extract it to a directory on your desktop named iPhoneTool
Code:
LINK: http://rapidshare.com/files/43146020/iphonetool.zip

3.) Open the program “Terminal” location in HardDrive/Applications/Utilities

4.) Type the following command into the flashing cursor:
cd ~/Desktop/iPhoneTool

5.) Now make sure your iPhone is connected and you quit iTunes

6.) To activate your iPhone type the follwoing command into the flashing cursor (note that there are two dashes before activate):
./tool –activate a.plist

7.) Your iPhone screen should say something about a SIM error which is fine. Just slide the unlock bar over and it should work. Now connect it to your computer and you should be able to configure it through iTunes.