while we're here again, I think I'll go ahead and mention this super easy super fast way of switching from KDM to GDM by default.
First you need to know if you have both installed. If not, install the one you don't have.
sudo apt-get install gdm
to install GDM.
sudo apt-get install kdm
to install KDM.
If you already have both installed then reconfigure one of them (it doesn't matter which one) like so:
sudo dpkg-reconfigure gdm
Now, it doesn't matter which of the three above mentioned methods you used, you will be presented with a dialog asking you which default you want to have (if, and only if, you have successfully installed more than one). Make your choice.