I've been having the same problem. The yum repositories did not transfer correctly when I upgraded from F10 to F11 either, which wasted over a day of my time
For the record, I didn't upgrade via yum; a window popped-up asking if I wanted to upgrade to a stable release of F11. A couple of mouse clicks and an hour later I had F11, but yum didn't work...
The poptop-stable repo is installed with pptp, and can be found at:-
I'm not a guru so didn't delete it (or use yum --disablerepo), but I changed the file to read: "enabled=0", and then the problem went away.
At this point I created another problem though. I'd disabled nearly all of my repos, making yum update not do anything. So I re-enabled all except pptp.repo, and the problem reappeared.
I've been looking for a default F11 repo list, but to no avail.
However, you should disable all the rawhide repos, as they try to update to F12 releases.
i.e. change enabled=1 to enabled=0 in the following files:
and this one too:
Hope that works for anyone else with this problem too...