SLES 10 SP2 hung on shutdown (any of 'shutdown -r now' or 'reboot').
Before this, this symptom had appeared:

The shutdown completed fine, but the following messages appeared on the screen:

Starting installation
The previous installation has failed
would you like it to continue?

Note: You may have to enter some information again.

[Yes] [No]

If you choose No, the boot process completes successfully.
If you choose Yes, it start the second phase of the installation procedure (asks to set the root password, first user, etc.)

I have choosen No and now I have reset the server in order to succeed the booting process.
No problems with SLES 10 SP1, although it is installed onto ESX VMware.

Sorry, didn't spot the cause yet. Opening a case at Novell.