When upstream released 5.1, everybody wanted to test a new kernel parameter that could adjust the system clock rate at boot time to something else than the standard 1000Hz clock rate. A lot of testings has been done (thanks to Akemi Yagi for her great work) and you can see the results here : http://bugs.centos.org/view.php?id=2189
As you can read at the bottom of the comments, it seems it was a typo in the official RH Release Notes : you'd have to read divider= and *NOT* tick_divider= ! (see http://www.centos.org/docs/5/html/release-notes/as-x86/RELEASE-NOTES-U1-x86-...)
It seems so to work with the correct kernel parameter and so there is no need to build a kernel-vm for CentOS 5.1 guests .. (it's still needed for example for 4.x ..)
Keep on reading the comments on bugs.centos.org for further informations ... I assume that upstream release notes will be corrected to reflect the real parameter