My wife's computer has the same configuration as mine: openSuSE 11.1 and KDE 4.3.2.

Sometimes, when she switches between applications using Alt+Tab KDE shuts down and KDM greeting dialog appears.

How to get some additional diagnostics? Can usual KDE crash report be created in such situation?

You need to grab the ~/.xsession-errors file immediately after being thrown out to the KDM logon screen.

You can do that by logging in as a different user then copying the file from the previous users /home/<previous user>/ folder.