There are quite a few bug reports on the kde bug tracking system about kde losing display settings on logout (e.g. Bug 183143). But since this problem has been present for a couple of years and is still existing in the latest version of KDE (i.e. 4.8.5 at this writing), here’s a workaround:

Add a parameter called StartupCommands to the krandr configuration file (usually located at ~/.kde/share/config/krandrrc that adjusts your display layout using xrandr on login:

[Display]
ApplyOnStartup=true
StartupCommands=xrandr --output "HDMI-0" --primary --output "VGA-0" --right-of "HDMI-0"

You also need to make sure that the command is invoked on login by setting the ApplyOnStartup parameter to true.

The actual xrandr command may of course look different, depending on the actual number and layout of screens. So YMMV!