The creator of Big Brother Camera Security has use his app to annonoismly collect what people are using as their passcodes to log in.
He reported that the ten most common passcode are:
1234, 0000, 2580, 1111, 5555, 5683, 0852, 2222, 1212, 1998
So if your passcode is one of those you should change it.
This is his graph that shows which of the ten is most common.More information about his findings can be found here:http://amitay.us/blog/files/most_common_iphone_passcodes.php