In the words of an Orange spokesperson...
One of the features of our devices is the programmable security engine on the platform. This is done to protect the ROM and the boot loader from corruption or from being overwritten. All production devices are secured utilizing keyed encryption.
For these reasons, the San Diego can’t be unlocked, and we don’t have any future plans to offer the ability to unlock it.
This confirms what I have seen in my research to date - the device will not boot a modified boot or recovery image. This also suggests that if we DO get root via an exploit and manage to write the Superuser files to the system partition, the device then may also not boot if the main system partition is being checked too. We have seen numerous Ice Cream Sandwich leaks for the devices (engineering releases with Superuser access), but these do not flash to retail devices, suggesting test devices have unlocked bootloaders and recovery partitions that flash test signed update zips.
This news will be quite a disappointment to those (myself included) who hoped the San Diego would succeed the San Francisco as the enthusiasts phone of choice. Aside from messing around with ROMs and the like, a number of applications I use on a daily basis NEED root access so the phone really isn't for me (to the extent I now have to decide whether to sell it on).
The big manufacturers have learnt now that this isn't the way forward and giving consumers choice is the way to be successful... it seems Orange have yet to learn this (I was going to write 'the networks' but kudos to Vodafone for shipping the Ascend G300 with an open configuration).
A final thought for Orange / Intel... although you've locked down the phone, you haven't done a great job. A user with a basic (dangerous) level of fastboot knowledge can 'fastboot flash' both the boot and recovery partitions, effectively 'bricking' the phone.
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