Remember the major security threat introduced with 2.0.2 which allowed anyone to access all the major phone application even if you had a passcode?
Apple said that they fixed the security. But, actually security not completely fixed. Another major bug still exists (one that has been around forever). The Emergency Call mode in passcode screen which is just meant to call emergency numbers like 911 can actually be used to call any number!
This is a big flaw as anyone can make a call and make you pay a big bill if somehow he/ she gets the phone! This means that iPhone Passcode is a big failure as anyone can access the main feature of iPhone really easily even if you think you have secured it.