A sharp-eyed reader notes that my failing case isn't connecting to ioapic0, and 
suggests that I may need to boot from a kernel with the ioapic bits enabled.

Yes, this is an SMP box.

I see that my 3.0.1 iso image has a GENERIC.MP.gz kernel, but I don't know (and 
the docs don't tell me) how to boot with that kernel... dmesg tells me that this 
is an INSTALL kernel, not INSTALL.MP ...