Laptop is back! Wow, that's about 48 hours turn around. Apple++
Repair note just says: Unit Diagnostic, PCBA,MLB,700MHz, Full Unit Inspection and Test.
I assume that means I was right - it was a cracked motherboard and they replaced it.
It's come back all shiny and clean, and with OS 10.2.6 re-installed for some reason (I only know this because iTunes prompted me with the license, and sudo gave me it's "Don't be a luser" warning. It's also been screwed together better, and doesn't twist or creak any more.
Had to re-add in my open firmware hack to allow extended desktop to work.
All looks good so far.