Some time ago there was a discussion on the above subject. I have scanned the past few month's mailing list archives and cannot find the relevant mail(s).
Could somebody please repost the solution or point me at the correct resource.
What you want is:
# yum instal yum-utils
followed by:
# package-cleanup --oldkernels [--count=x]
where x defaults to "2" (i.e., keep two older kernels).
Alfred