I'm happy to report that the updated Gitco Xen 3.4.1 repo is working well on CentOS 5.4.
If you are doing link bonding and bridging in accordance with my previous post "Xen 3.3 in RHEL/CentOS 5 and more Link Aggregation Fun", you no longer need to patch the network scripts as RedHat fixed the initscripts package in RHEL 5.4.
Upgrade procedure for CentOS 5.3 to 5.4:
yum clean all yum update glibc\* yum update yum\* rpm\* python\* yum clean all yum upgrade reboot
Updated Xen Install Guide From My Previous Article:
Head over to http://www.gitco.de/repo/ and grab the repo for your arch. (Most likely wget http://www.gitco.de/repo/CentOS5-GITCO_x86_64.repo in /etc/yum.repos.d/ for the uninitiated).
If you already have Xen installed, you may need to remove and readd it.
yum groupremove Virtualization yum groupinstall Virtualization
You'll also get some updated tools like Virtual Machine Monitor 0.7.0 that make it easier to install newer guests such as Fedora 11 or Ubuntu. Sweet!
Double check /etc/sysconfig/kernel. It should be set to kernel-xen. Likewise, check /boot/grub.conf and make sure that the Xen kernel is the default if the aforementioned was not done beforehand.