According to http://wiki.centos.org/Manuals/ReleaseNotes/CentOSLiveCD5.5 "There is a "Network Install" option on the Live CD that is the same as our CentOS-5.5-i386-netinstall ISO".
I've looked quite carefully at my CentOS-5.5 Live CD (on a USB stick), and I don't see a Network Install option anywhere.
Could some kind soul explain where it can be found, please.