Fixed by http://xenbits.xensource.com/staging/linux-2.6.18-xen.hg?rev/877c2e42a701
This affects the RHEL5 and RHEL45 kernels.
You can download this test kernel from http://people.redhat.com/dzickus/el5
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on the solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.