From the Get Virtualized dept.:
Red Hat today officially announced the beta availability of Red Hat Enterprise Linux 5.4 (RHEL), which in my view is a lot more than a typical point release. Sure we're all waiting for the big RHEL 6 release, but there are some major changes in RHEL 5.4.

The most obvious change is the shift to the KVM hypervisor (as opposed to Xen). Xen is still in RHEL, but with RHEL 5.4, Red Hat is signaling its intention that KVM (eventually) is to be Red Hat's preferred Hypervisor. It's a preference that Red Hat execs have indicated at multiple points this year and should be no surprise since Red Hat now owns lead KVM vendor Qumranet.