kinfauns and pokey9000 over at XDA-Developers have confirmed that the software updates imply that the MLO (xloader, 1st stage bootloader) is signed and the boot header is the type used for HS (high security) OMAP devices with the M-Shield turned on.
What does this mean to the average user like you and me, who would like to install a custom rom or even root? Initially at least, we can't. This is a similar situation to the Nook Tablet which took serious effort to beat.
It has been suggested previously that Amazon are making a loss on selling the hardware at the prices mentioned, one way to retain a customer is to lock the device down so that they can use the Amazon App Store.
The devices are not available in the United Kingdom until the 25th October 2012, so if you have one of these on pre-order we will let you know if a way to unlock the bootloader becomes available.
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