I read recently in an article that you can install kubuntu on a ubuntu install by choosing kubuntu-desktop. Now I use kubuntu because I tend to prefer KDE, but I thought it would be interesting to see if you could do the inverse. So I started up the adept package manager, and search for everything -desktop. I found that you can install edubuntu-desktop, ubuntu-desktop and kubuntu-desktop. I chose ubuntu-desktop.
The installer ran, but it seemed to get stuck on preparing to install. It sat their for a while, as I worked on other stuff, for about an hour. After an hour I figured something was wrong. Choosing details showed me that the installer was actually waiting on input from the user, you had to choose whether you wanted to use gdm or kdm. Their should be an easier way to tell if the installer is waiting for input from the user. But in any case, if you try to install a package in ubuntu/kubuntu you might want to check on the details if the installer seems stuck.
You could ofcourse just do sudo apt-get install ubuntu-desktop, from the command line, which would show you if the command line installers are waiting for input.