Version: (using KDE 4.0.2) Installed from: Debian testing/unstable Packages Sometimes the Alt-Tab combination stops working in the KDE session. Both keys work individually, just the combination has no effect. It happens once or twice a day so it's quite annoying. In such cases Ctrl-Alt-Del also has no effect, but I can e.g. bring up a File menu by pressing Alt-F. Debian bug report: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=470625
Seems to work in later snapshots (Debian 4.0.72).
If this recurs in the future it is probably due to kded4 crashing (which detects the keystrokes in question and activates the appropriate modules). If this is the case try running konsole or some other terminal and seeing if running kded4 fixes the issue.