If you need to obtain your IMEI # for some reasons, here is how. No, you don’t need the original box of your phone. ;) BTW, I am using an iphone 4 with at&t.
Fire up your phone app, then go to the Keypad tab, then punch in *#06# and a screen should automatically pop up with your IMEI #.
Actually… upon more research, for iphone 4 users, it’s actually a lot simpler. Just go to Settings -> General -> About, then scroll down to find out your IMEI and ICCID #s.