It was a test kernel and is no longer available. Test kernels are not part of the release cycle and are not available for installation via yum.
See for yourself: http://people.redhat.com/dzickus/el5/
Warning install any of these test kernels is not a good idea.
The released kernel-2.6.18-8.1.8.el5 should have the fix included if it was acceptable since it was released well after May 9th.