No, the Mirabox wasn't seeing the card for some reason. I've tried again with another SD card, and it's seeing this one, so I'm able to get a bit further. I can now get the kernel to boot, but it hangs almost immediately after the line that says "[ OK ] Found device /dev/ttyS0." For what it's worth, this is the same place my OpenBlocks AX3 is hanging as well -- so I'm wondering if this is a problem across all the Armada-XP systems.