The last YUM update (dated aprox 11/2/06) included a kernel update. After install on my system GRUB did not boot the newly updated kernel, instead it continued to boot the old kernel. (This is probably not the case on all systems but it bears checking.)
This can be checked using the following command.
[root@sme7 ~]# uname -ashould return
Linux sme7 2.6.9-42.0.2.ELsmp for a multiprocessor system.
or
Linux sme7 2.6.9-42.0.2.EL for a single processor system.
See bug report here
http://bugs.contribs.org/show_bug.cgi?id=2061