Somehow my FC-6 machine had fstab containing: /dev/proc /proc proc defaults 0 0 This caused this error box in anaconda after choosing Upgrade: Unable to mount filesystem An error occurred mounting device proc as /proc. You may continue installation, but there may be problems. I have no idea how /dev/proc ended up in fstab, but Google seems to indicate many other cases of this being valid. Perhaps this is a corner case we should handle?
Please try anaconda-11.3.0.29 and let me know if it's working better for you.