Dear readers,
anyone having idea how to turn it off? Previously with kde3.5 it wasn't that way, now it is. (a small window pops up and counts from 30 to 0 to log out -- what an annoyance!)

You need to change the appropriate profile setting in System Settings > Advanced > Power Management > Profiles.

Thanks, but is there any chance to set it to "do nothing" as default for the new users of system?

This should be possible. You need to copy powerdevilrc and powerdevilprofilesrc from ~/.kde4/share/config to /etc/kde4/share/config

This config will only be applied to new users. If you wish to apply it to all users, simply remove the powerdevilrc and powerdevilprofilesrc files from their ~/.kde4/share/config folder.

It works! Just a note on this: if I move instead of copying, thus getting no power* files in ~/.kde4/share/config, there's trouble. On what event are the user power* files created, presumably taking those of /etc/kde4 as template?

They will be created when a user logs into KDE, as that is when KDE Daemon, and thus, the Power Manager, PowerDevil, is started.