If you install a KDE application from the repositories in Gnome and afterwards
check your installed packages using the filter or if you do a search for
kde-core in Synaptic, then you will find that kde-core has been installed since
it is a necessary dependency. Once it is installed then it will not need to be
installed again and other KDE applications will install. Some other KDE
applications will need more of KDE in order to operate and as you add
applications you will eventually end up with enough KDE to be able to run it
from the session manager.

I found this out when I was using Hardy last January or December when it was
very experimental and it included KDE 4 for the first time. I could not get KDE
4 to install because of dependency problems between kde 3 and kde 4. I wanted to
see what some of the applications were like, so I began to install them
individually, those that would install.

I was surprised one day to see that KDE 4 appeared as one of the window managers
under session at login. So I changed the session to KDE 4 and it loaded, but
without full functionality. I had no taskbar or configuration, but could add
widgets and run programs. Eventually the dependency problems sorted themselves
out and I was able to install the full KDE desktop with all of the bells and
whistles.

What had happened was that update manager would present me with a list of
updates. Most of them were ones already on my system, but occasionally one would
appear in the list that said, not installed. If I told it to update with
de-selecting it, it would add a piece of KDE 4 and as the pieces began to grow
and KDE 4 began to take shape.