The former installonlyn yum plugin is now built into yum.
You must add the variable installonly_limit = # (where # is
the number of kernels you want to maintain installed
when a new kernel is installed) to your yum.conf file in CentOS 5.2.
The default if nothing is added is 3 kernels.
This will need to be updated even if you have the current
plugin turned off or set to a different number as
it does not happen automatically.
# PUT YOUR REPOS HERE OR IN separate files named file.repo
# in /etc/yum.repos.d
#the number of kernels