Could the security on the boxes be too strong? Unlikely I'd say.
You'd actually be suprised. A lot of the chipsets now on boxes have secure bootloaders on chip in the CPU, which then check a signed bootloader in the flash.
If these dont add up, then the box does not boot. Very similar to the Xbox360 security system.
BT Vision or rather sigma designs (SMP8634) has the same thing in their chips also.
The thompson skyhd box also has a similar system as it uses the BCM7038 chip which supports secure boot, altho its not actually implemented in that box.
Mick