Had a problem with installing sogo on centos 5.6. The yum installer complained that there was a missing dependencey for the memcache. Like this.

memcached-1.4.2-1.el5.x86_64 from sogo-rhel5 has depsolving problems
  --> Missing Dependency: libevent-1.1a.so.1()(64bit) is needed by package memcached-1.4.2-1.el5.x86_64 (sogo-rhel5)

The problem was resolved when the following package was installed.

compat-libevent-11a-3.2.1-1.el5.rf.x86_64.rpm from

http://pkgs.repoforge.org/compat-libevent-11a/compat-libevent-11a-3.2.1-1.el5.rf.x86_64.rpm