Apparently works on CentOS when you are trying to run an update/upgrade and dependency issues pop up. Don't know if it works under all circumstances - but definitely worked for both Shahbaz Javeed here and for yours truly just a few minutes ago. Like Shahbaz, I am also running CentOS 5.3 on the machine in question.
What you do is:
# yum clean all
Yes, it was this simple - at least this time!