Version: 4.2.2 (using KDE 4.2.2) OS: MS Windows Installed from: MS Windows try this demo: http://pc44.one.pl/goorol/bugs/LastModified-konqueror.php
This behaviour is still present in a recent SVN snapshot r984032, confirmed using wireshark and page linked to.. I believe this behaviour is desired though. When you press F5, you are refreshing the page, and thus refreshing the cache. As such, this parameter is not needed in the request.
This is by design: When you press F5 the browser and KIO try everything possible to get the most recent version. This includes ignoring cache contents. if-modified-since headers are only good to make sure that the cache contents are still current, but we don't want to use them anyway, so...