Xserve CPU Status LED's under Mountain Lion
It is possible to re-enable the front-panel LED array on the Xserve when running 10.8. It appears that the process responsible for displaying/updating the readout on the front panel is the hwmond process. Unfortunately, hwmond is not included with OS X v.10.8. However, it is possible to use the hwmond executable from a 10.7.5 system.

The hardware that I tested with was an Early 2009 Xserve. After copying hwmond from the 10.7.5 system into /usr/sbin, I copied the com.apple.hwmond.plist launchd configuration file from /System/Library/LaunchDaemons/com.apple.hwmond.plist, and installed into /System/Library/LaunchDaemons on the 10.8.5 system. After a reboot (or a launchctl load -w /System/Library/LaunchDaemons/com.apple.hwmond.plist), the system identifiers are working normally.