On my laptop with an ATI graphic card and kubuntu installed I am using either a dual-head configuration in xinerama mode (screen stretched over two monitors) or a simple one monitor setup.

Recently I messed around with the KDE system settings to get a projector working during a presentation. At home again, I wanted to run X in dual-head mode as usual and replaced the xorg.conf with the backuped xorg.conf.xinerama.
Big was my surprise that despite of this kde started in clone mode. So somehow there must be settings outside the xorg.conf file hiding itself in a cosy corner of my file system. Starting up KDE as an other user would show the correct screen.

Where KDE stores this user-related dispaly setting?

~/.kde/share/config/displayconfigrc

If you have got similiar problems – remove it and everything will be fine again!