Root's umask is set to 077, so files created by root are by default only
readable by root. Safest setting, so quite ok.
Now root runs an rpm or yum command to install or update files. Umask
setting is not used for the files extracted from the rpm, those get the
permissions as stored inside the package. 
But root's umask appears to be in effect for the scripts that run from the rpm. also, directories not owned by the rpm are created with the default permissions.

Most notably: various files in /usr/share/mime become inaccessibel to
normal users after root manually installs updates, if those updates call for one of those tools that update the mime bindings. 
Similarly, after the last update of tetex, files like /usr/share/texmf/ls-R were only readable for root.

If I explicitly execute 'umask 022; yum upgrade', there is no
problem. So I think this is a bug in yum/rpm.

Actually, I have been having problems with files like those in
/usr/share/mime for a long time, probably since Fedora 8 came out, I
just never correlated the problem to the way the updates were installed
until now.

On Fedora 9, I have not been able to reproduce the problem, even though
the umask settings are the same.

PS: umask is set in /etc/bashrc, which is unmodified on these systems as installed by the setup package