Also, thanks for the link -- it describes a kernel panic based on a bad video module, but it affects all Arch installations on all RPi platforms.
The thing is, as the serial/UART connection is the primary output, I *usually* get all errors sent via the serial line to screen runnning on my laptop. With no serial activity, I can't tell if there's a kernel panic or not -- usually *something* gets sent. This is an unusual situation: I can't diagnose 'is there a kernel panic?'.
Since the errors seem to be cropping up for all RPi platforms, I think this answers my question on another thread "Is Arch on RPi Broken?" The answer seems to be 'Yes'.