USB controller issues? We used to have these really hard-to-debug problems 
with the USB controller being active and having had the BIOS set up the 
command queues etc. Really subtle. It's why we now have PCI quirks for 
shutting up (most) USB controllers very early.

On powermacs or powerbooks, the USB controller is shut down by the
firmware when we call the "quiesce" OF call from prom_init.c, which
happens before the kernel relocates itself to 0 and takes over memory.
Unless we fucked up something in there, I wouldn't expect that to be the
cause.
 

If there is some USB controller that we miss, or that sets up its command 
chain to some unexpected area (so that USB is active and corrupting memory 
even very early on), that could explain it.

Did we setup the OHCI controller when the crash happen ? Maybe we broke
something subtle in the USB stack ?

Ben.