Hi there,

>Presumably this means one (or more) of its SIMMs is about to fail? 
>Is it possible to tell which one(s)?

Maybe, in OBP setenv test-megs# ( or something like this, it should be set to 1 so OBP checks just the first megabyte on power up ) according to your 
installed memory, cd /memory and selftest. Maybe OBP sees the defect too and it can possibly tell you which socket holds the defective module. 

have fun
Michael