As Fabian Deutsch noted recently, some F16 users might be seeing yum somewhat inexplicably failing to download repomd.xml for the Fedora repositories, which will stop you being able to install updated packages. Seth Vidal mentioned in the comments that this was caused by a bad nss update.
To complete the information, the offending build of nss is nss-3.13.1-2.fc16. It was submitted as an update but then retracted when the bug became apparent, which means it doesn't show up in Bodhi's interface any more. If you have this build of nss, you should manually downgrade all installed nss subpackages - nss, nss-devel, nss-pkcs11-devel, nss-sysinit, nss-tools - to nss-3.12.10-7.fc16, which should resolve the issue and allow you to use yum as normal from there on. You will only have been affected by this issue if you had updates-testing enabled and updated while the bad nss update was available, it never made it to the stable update repo.