When Red Hat 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 by the CentOS QA Team and you can see the results here : http://bugs.centos.org/view.php?id=2189 (Notice that Xen guests don't need the system clock rate to be modified because they already have a 250Hz kernel)
As you can read at the bottom of the comments, it seems there was a typo in the official RH Release Notes : you'd have to read divider= and *NOT* tick_divider= !
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 ..). The CentOS 5.1 Release Notes have been corrected to reflect the good divider= option .
Here is a small benchmark of an idle minimal CentOS 5.1 i386 running in a VMWare-server guest VM without (before 20h40) and with the divider=10 option (after 20h40 , so system clock rate fixed to 100Hz) : You'll notice directly that from a Host point of view, the vm consumes less CPU than before the divider option :