In fedora 15 default Display Manager when installing both KDE and Gnome is GDM. Gnome3 is near sexy to KDE4 but GDM really sucks.
To switch from default GDM to KDM in fedora 15 do as follow
su -c 'echo "DISPLAYMANAGER=\"KDE\"" >> /etc/sysconfig/desktop'
su -c 'echo "DESKTOP=\"KDE\"" >> /etc/sysconfig/desktop'
You can also try an alternate method by opeing the file /etc/sysconfig/desktop
then add the following lines or make appropriate changes if they exist