Yum installonlypkgs purging all but previous version of kernel

[Ed]

I would like to keep the current and previous 2 kernel releases, in
FC5 the installonlypkgs list did not remove packages in the list and
(ISTR) I removed old ones manually.

In FC6 I have kernel and kernel-devel in the installyonlypkgs list in
yum.conf but after a kernel upgrade I find that all versions older
than the previous have been removed, e.g. today I upgraded to kernel
2869 and now only have kernels 2868 and 2869 on my machine. No package
removal is listed on the yum log

How do I get the previous behaviour?, ideally specifying the number of
versions to keep.

Ed