If you are using an iPhone running iOS 6.1, be warned, your passcode can be easily bypassed. As of right now there is no way to secure your device. A similar hack existed in iOS 4.1 and it took Apple nearly a month to issue a fix. This hack also requires the dialing of an emergency number and an immediate hangup which is illegal.
Keep in mind this hack requires physical access to your phone and someone with good timing.
You can see more about this hack via this link from Jailbreak Nation.