Then yesterday's update came and it hasn't rebooted a single time since. It's a bit early to know for sure but it does seem like the issue is gone.
I don't think it's a coincidence that AT&T is now offering "certified" cards. The cards didn't change nor did their level of testing... Samsung just fixed the underlying problem. Microsoft has shown that even their certified cards can vary greatly; it's just a product of the manufacturing process.
I will note that that I put the registry changes back to their original state (DisablePowerManagement=1) as that is how the firmware that shipped yesterday has it set.
Anyone else care to chime in? Cautiously optimistic that these absurd SD card issues are behind us.