Different distributions such as Debian and Centos behave differently when trying to shutdown your system.
shutdown -H now on Debian does not do what you'd expect. The system won't power down, it will halt and do everything but power down.
shutdown -PH now will do the job though (actually power the system off). This is important to test especially if you are not near the system. If you just use -P it forcefully shuts off which is not good since it won't stop the services or even unmount drives/arrays etc..
shutdown -hP now
Broadcast message from root (pts/0) (Wed Aug 25 02:04:08 2010):
The system is going down for system halt NOW!