The advice to rebuild the RPM DB was incorrect and not helpful. It doesn't fix these errors.
Run yum distro-sync
and afterwards show the output of yum list libcom_err
as for some reason you only get the i686 build of libcom_err-1.42.3-2.fc17 and not the corresponding x86_64 package. That's unusual.
# yum list libcom_err
Loaded plugins: auto-update-debuginfo, langpacks, presto, refresh-packagekit
libcom_err.x86_64 1.42.3-2.fc17 @updates-testing
libcom_err.i686 1.42.3-2.fc17 updates
About how to remove i686 packages, look into using "rpm" or "yum remove …", but be careful.