Apple recently released iOS 6.1.1 to repair a problem affecting some iPhone 4S devices, which left the devices with no voice or data connectivity. Vodafone even warned their iPhone 4S customers against upgrading to iOS 6.1 to avoid them loosing access to making calls, sending texts or browsing the Internet.
Now a new problem has been discovered on iOS 6.1 involving the pass-code lock.
A bug in the software version allows someone to break into a pass-code protected Apple iPhone running iOS 6.1 in just a few easy steps
So until Apple finds a way to fix the newly discovered issue your iPhone is vulnerable to be unlocked even when you have a passcode lock on, and we will have to wait for Apple to release a potential iOS 6.1.2 update to fix it.
source – GSMArena