I have had similiar problems:
--> Package fping.x86_64 0:2.4b2-7.el5 set to be updated
--> Processing Dependency: nagios-plugins = 1.4.15-2.el5 for package: nagios-plugins-load
The origin of the problem was an additional package repository, in my case 'Dag RPM Repository for Red Hat Enterprise Linux'. I have created an extra configuration file /etc/yum.repos.d/dag.repo
To check if you have the same problem, execute the following command
yum list nagios\*
* epel: mirror.fraunhofer.de
nagios-plugins.x86_64 1.4.15-2.el5 installed
nagios-plugins.x86_64 1.4.15-2.el5.rf dag <-- the additional repository!
The solution is to exclude this package in the dag-repository. You can do this on repository level.
Add the following line to the additional repository configuration file(/etc/yum.repos.d/dag.repo):
Note: If you want to exclude more packages just add them to the line with a space between different excludes.