This sounds like a known design limitation in KDE 4, which doesn't support multiple X screens. I don't know why it would have worked for you in Hardy, unless you also went back to KDE 3. The code is just plain missing from KDE 4.
Thank you for the reply and for the explanation. You are correct in assuming I was using KDE 3 in Hardy.
Perhaps future releases of KDE 4 will include the code for multiple X screen support?
All the best,