ALT+CTRL+F1-F6 quit working
As should have been clear from the original question, ALT+CTRL+F1 switches to a text console, but only before logging in to a KDE session. Once a KDE session is open, ALT+CTRL+F12 followed by ALT+CTRL+F1 works. This is therefore not a problem with /etc/inittab (which has a date of 7 Sep 2007, much older than the onset of the problem), or X (xorg.conf has a date of 14 Dec 2008, also much older than the onset of the problem). The problem is almost surely a KDE problem. I can't find anything in the KDE Control Panel that re-maps ALT_CTRL_Fn (n=1..6).