The mysqld in the 4.0.3 install (and probably others) is statically linked, which is great for most installs. But XEN installs require linking against a nosegneg libc version -- which is usually taken care of by dynamically linking against libc6. I found lots of information on nosegneg and XEN, but do not pretend to be an expert. I was chasing down the issue of the message "4gb seg fixup, process mysqld" which appears ins /var/log/messages.
My system appears to be running, but I am having some strange behavior with users not authenticating and ldap stopping.
Although in part I'm guessing, I believe that linking against a nosegneg version of libc6 will fix these problems and such a version will run in the non-XEN environment also. With some CPU's this may not be necessary -- namely the CPU's with the virtualization extension that allows Windows to run in XEN. I haven't dug deeper, but I suspect most of the exec are statically linked? I would hate do a source build, but I'm not afraid of it either.
Regarding my own environment, I am running Zimbra in a XEN virtual on FC5 with the latest patches for both the host and the guest OS. Both host and guest are FC5.
Again, I am just guessing about half of this -- but I'm not guessing about the other half.