I have just noticed that Red Hat added Ext4 support to RHEL-5 in kernel 2.6.18-110.el5. They also added a new package named e4fsprogs (a break from the e2fsprogs name that has been used for so long). Hopefully they will use a single package for utilities for Ext2/3/4 filesystems in RHEL-6 and not continue this package split. Using commands such as e4fsck and tune4fs is a minor inconvenience.
Converting a RHEL 5 or CentOS 5 system to Ext4 merely requires running the command “tune4fs -O flex_bg,uninit_bg /dev/WHATEVER” to enable Ext4 on the devices, editing /etc/fstab to change the filesystem type to ext4, running a command such as “mkinitrd -f /boot/initrd-2.6.18-164.9.1.el5xen.img 2.6.18-164.9.1.el5xen” to generate a new initrd with Ext4 support (which must be done after editing /etc/fstab), and then rebooting.
When the system is booted it will run fsck on the filesystems automatically – but not display progress reports which is rather disconcerting. The system will display “/ contains a file system with errors, check forced.” and apparently hang for a large amount of time. This is however slightly better than the situation on Debian/Unstable where upgrading to Ext4 results in an fsck error on boot which forces you to login in single user mode to run fsck  – which would be unpleasant if you don’t have convenient console access. Hopefully this will be fixed before Squeeze is released.
I now have a couple of my CentOS 5 DomUs running with Ext4, it seems to work well.