On a modern distribution, OpenLDAP (and other software using NPTL) should
work without problems. The requirements are:
-Xen 3.1 hypervisor with PAE (older versions may also work)
-Recent-enough glibc (2.4, or 2.3 with the correct patches)
IOW, RHEL5/Centos 5 should work fine. In my experience this is the case.
I fired up OpenLDAP 2.4.7 with db4.6 (with NPTL) on a RHEL 5.1 Xen DomU
(I don't currently have a working RHEL5 Dom0) this morning without
problems, and in the past I have run a more complete setups on Xen Dom0s.