[CentOS] NMAP - reveal MAC address

Wed May 7 16:07:17 UTC 2008
Tom Brown <tom at ng23.net>

In CentOS 4 does anyone know the switches to get NMAP to reveal the MAC 
of the host being scanned ?

I cant seem to find it and i am using nmap-4.20 - i am sure this was 
available somehow on older releases.