After receiving a number of helpful suggestions, I managed to boot 
the Q700 once using a new 1.4.2 kernel on the Mac partition.  That 
ran stably for a day or so, then I tried to reconfigure things to get 
Ethernet up and running (what a head spinner that is).  Anyway, when 
I rebooted the system it would not boot.  The boot process stopped 
again at the

adb0 at obio0

line.  Letting it sit for a while it seemed to do nothing, and then 
broke into a rapidly cascading series of error messages.  Squinting 
through the blurs, I caught the following:

kernel: MMU fault trap
and something about "exception in adb"

Is this indicative of a hardware fault?  Broken PMMU emulation in the 
CPU or something?  Loose wire somewhere on the motherboard?

Thanks!