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..

For Centos 5.5 here is how you power down

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!