RHEL 5.5 updates libevent to a package with a new library version (and doesn't provide a compat package - so much for RHEL's ABI stability). This breaks at least memcached from EPEL. However, the new libevent is required by a new nfs-utils.
I guess the "fix" is for EPEL's memcached to be rebuilt (and then only work on RHEL 5.5+ systems).
epel-devel@lists.fedoraproject.org