Has iOS Always had Secure Boot?

In Apple's recent iOS Security paper here http://images.apple.com/ipad/business/docs/iOS_Security_May12.pdf

They speak about the Secure Boot Chain, my question is has iPhones/iPods/iPads always had this feature?

"Secure Boot Chain Each step of the boot-up process contains components that are cryptographically signed by Apple to ensure integrity, and proceeds only after verifying the chain of trust. This includes the bootloaders, kernel, kernel extensions, and baseband firmware"

Solution 1:

The Secure Boot Chain is a succession of steps designed to check and verify the integrity of the iOS. As it ensures too that the iOS runs only on validated Apple device, I'd say that it has been like this since the beginning.