Mariusz W and Wong Kwok-hon were having trouble with the poweroff command only halting their machines.
I can't see any problems looking over your /etc/dmesg outputs. Could you try booting with acpi=off on the kernel command line (assuming neither of you have hyperthreading or SMP)? That would help rule ACPI in or out.
After that, though, I think you're going to have to bugzilla it.
Sorry I can't help more.
James.