Are you sure? I've read on a few different sites / emails that people do this, mainly because the 64bit version can handle the large amounts of memory typically found in dedicated servers.
http://www.redhat.com/docs/manuals/enterprise/RHEL-5-manual/Virtualization-e...
"With Red Hat Virtualization, 32-bit hosts runs only 32-bit paravirtual guests. 64-bit hosts runs only 64-bit paravirtual guests. And a 64-bit full virtualization host runs 32-bit, 32-bit PAE, or 64-bit guests. A 32-bit full virtualization host runs both PAE and non-PAE full virtualization guests."