I had the same "problem" too when installing Fedora Core 5. For some reason, the graphical interface doesn't start up automatically like it did with previous distributions. Basically, you need to login with the Root account, which you should have set up during installation. After succesfully logging in, type the command "startx" and it should load the GUI.
On sort of a related problem, the command "startx" starts the Gnome gui automatically. Does anyone know how can I switch it to start KDE instead?