Why doesn't the FBI simply clone the current device, make brute force attempts and then clone again if locked out? Yes, lots of work but also doesn't force Apple to participate.
The iPhone uses AES encryption which would prevent cloning the flash storage [1]. There was an informative discussion of this over on AppleInsider - http://forums.appleinsider.com/discussion/191851
That's what the A7 (iPhone 5S and later) design does:
“Each Secure Enclave is provisioned during fabrication with its own UID (Unique ID) that is not accessible to other parts of the system and is not known to Apple. When the device starts up, an ephemeral key is created, entangled with its UID, and used to encrypt the Secure Enclave’s portion of the device’s memory space. Additionally, data that is saved to the file system by the Secure Enclave is encrypted with a key entangled with the UID and an anti-replay counter.”
Yeah, it wasn't exactly unknown before but it wasn't terribly common outside of certain security / compliance circles. I think I've seen more links today than in the previous year.