I recently installed KDE 4.7 on my wife's computer. I did not use the recommended Debian 'install task' method, which emulates the full install-from-.iso process.
Instead I just did:
# apt-get install kde-plasma-workspace plasma*
She uses GNOME, and wanted KDE because apparently you can take advantage of some additional features in kdenlive when you are running it from KDE. And so I opted to keep both WMs going. Unfortunately, I was not prepared for the consequences of installing two WMs on the same machine. In this case, installing KDE rendered the GNOME desktop non-functional, though not GNOME itself, as in this question: Debian GNOME 2.30, desktop does not respond anymore.
Dolphin also made itself the default file manager in GNOME (gdm3), which was a headache. I removed Dolphin, and I have basically removed KDE, but it has not fixed anything of course.