From Fedora 11 Common Bugs:
When upgrading from an updated F10 install, yum ends up not working – outputting an error about no yum module found.
The version of yum in f10-updates was newer than the one in F11-GA – it caused yum to not be updated during the anaconda upgrade process. There is a simple one-time work around as root:
export PYTHONPATH=/usr/lib/python2.5/site-packages
yum clean all
yum update yum
That should update you to the current yum from F11-updates and you’ll be all set.
Update: I also had a strange problem with my user account, so I just made a new one then deleted the buggy one. All is good now.