updated 04:59 pm EDT, Wed March 20, 2013
Only affects iPhone 4
A new lockscreen bypass has been discovered in iOS 6.1.3, notes iPhoneinCanada. Unlike the tricks discovered in iOS 6.1, the new one appears to affect only the iPhone 4. It also, unusually, involves popping out an iPhone's SIM card while using Voice Control to dial an emergency number.
The hack was first exposed in a YouTube video, and has been tested by iPhoneinCanada. Once past the lockscreen, users can access contacts and photos. An easy way of disabling the hack, however, involves turning Voice Dial off in the Passcode Lock menu under Settings.
iOS 6.1.3, released just yesterday, was itself meant to disable lockscreen bypasses. It's believed to have solved both hacks in v6.1.