It's kinda scary how many microprocessor and different firmwares are needed/used in nowadays computer/hardware, and how each one of them add a new point of failure.
I was reading just today a similar article, but involving HDDs instead of Microsd cards (and even with a PoC): http://spritesmods.com/?art=hddhack
I have a few servers at work with one Intel CPU on the motherboard, but two PPCs sitting on RAID controllers, and about a dozen ARM controllers on the harddrives (that article scared me - consider patching the drive controller to modify your boot process by transparently injecting stuff into your reads; means it is insufficient to just reformat if someone gets root). Plus whatever CPUs handles the IPMI cards (monitoring/KVM/reboot functionality)
There might very well be more micro-controllers in them that I don't know about. And these are quite run-of-the-mill rack mountable servers...
I was reading just today a similar article, but involving HDDs instead of Microsd cards (and even with a PoC): http://spritesmods.com/?art=hddhack