Look in:
http://dev.centos.org/centos/5/testing/x86_64/
I think you should be using the testing repo.
yum list --disablerepo=* --enablerepo=c5-testing httpd
will find it.
Available Packages httpd.x86_64 2.2.8-1.el5s2.centos c5-testing
yep, things weren't where i expected to see them so i didn't even try yum.