This is a weird one. The bugger booted perfectly out of the box and seems physically perfect (awesome display, no yellow tint, no dead pixels, diagnostics OK), but something clearly went awry in the migration process. I nuked the HD from orbit a second time, booted from the install DVD, repaired the startup disk and was finally able to get up and running. Now migrating again from a separate backup volume. Fingers crossed!
BTW, this is my 4th iMac and first ever issue with going from one to another.