Today I also got the same message as the OP with my custom built Wings
computer. I tried upgrading from Fedora 13 to Fedora 15 using burned DVD installation disc (shasum was OK). After rebooting I deleted fedora-release-rawhide as advised by someone on this topic and after that I couldn't find Terminal and most of the icons looked like they were empty, so I couldn't do anything anymore with my skills.
I used the 'replace linux system' with CD-ROM installation disc (yes, I have two installation discs) and didn't try to remove anything. Instead, I simply commanded as mentioned on this topic:
And Fedora 15 seems to be working for me now.