In the manual for installing MongoDB from a yum repository there recommended to create a 10gen.repo file by hand. It is error prone and harded to script.
I would be better to provide:

• <a better version> a parametrized (currently only $basearch) 10gen.repo file and put in a manual only an instruction to get it into /etc/yum.repos.d directory (wget URL).

• <a much better version> a ready to install RPM file with packaged .repo file and (in the future) also a GPG key

See the "Command Line Setup using rpm" section in http://rpmfusion.org/Configuration