"I can tell you from the Department of Justice perspective, if that drive is encrypted, you're done," Ovie Carroll, director of the cyber-crime lab at the Computer Crime and Intellectual Property Section in the Department of Justice, said during his keynote address at the DFRWS computer forensics conference in Washington, D.C., last Monday. "When conducting criminal investigations, if you pull the power on a drive that is whole-disk encrypted you have lost any chance of recovering that data."
Technology Review explains that the original iPhone ran all applications with root privileges, giving each app complete control over the phone. Hackers could find exploits in those apps and take over the phone from there. But since then, Apple has invested lots in iOS security, and now each app runs in its own "sandbox."
Apparently, Apple is protecting customers' content so well that in many situations it is almost impossible for law enforcement to perform forensic examinations from seized devices.