It appears that the semantics of halt mean it will stop the machine but it may or may not shut it down. Back when I used UNIX boxes, halt basically stopped the machine but never powered it down; to power it down one had to explicitly use "halt -p".
So things change. With upstart, halt is a symbolic link to reboot and reboot calls shutdown -h. The man page to shutdown states for the -h option:
"Requests that the system be either halted or powered off after it has been brought down, with the choice as to which left up to the system."
Hrm, so this vaguely explains why halting some machines may just halt and on others it may also shut the system down. I've not digged into this thoroughly yet, but one suspects that for different processor architectures we get different implementations. Even for x86 we have variations in CPUs and boards/platforms, so it really it is hard to say if halt will power down a machine.
The best bet is to assume halt just halts and if you want it to power down always use "halt -p".