As been noted, the NetworkManager update in Fedora 9 and Fedora 10 prevents YUM from performing a regular update. Apparently due to a bug in the Fedora Update system. The following error may be seen:
Public key for NetworkManager-0.7.0.99-3.fc10.i386.rpm is not installed
The solution is coming soon, but for a temporary workaround YUM supports an exclude option:
yum --exclude=NetworkManager\* update
This should ignore NetworkManager related packages and continue on in the update process.
For more tips on YUM usage, run: yum --help.