Trying a new VM host that uses an ASUS motherboard. Unfortunately, old version of kernel in CentOS 5.5 means that no networking ever comes up. Specifically, the Atheros L1 Gigabit Ethernet driver isn’t in such an old version of the kernel. (Only came in in 2.6.21, centOS 5.5 is 2.6.18)
Fix: Going by the page here: atl1.sourceforge.net/ , grab ftp.hogchain.net/pub/linux/attansic/atl1/centos5.2/kmod-atl1-xen-2.1.3-1.x86_64.rpm and rpm -ivh it after somehow getting it to the new system – I just used a thumbdrive.