Version: 3.5.2 (using KDE KDE 3.5.3)
Installed from: Debian testing/unstable Packages
Compiler: gcc (GCC) 4.0.4 20060507 (prerelease) (Debian 4.0.3-3)
Konqueror caches favicon.ico per host, not per (host, port) as it should.
To convince yourself that per (host, port) caching is The Right Thing (tm), go to 127.0.0.1:631 (cups config), then go to 127.0.0.1:80 (your pwn l33t w3bs1te with a favicon.ico and all).
Confirmed this behaviour on KDE 3.5.5 / Kubuntu 6.10.
Confirmed behaviour still exists in 4.0.3/Kubuntu. Not that you may need to save a bookmark to get it to show the favicon in the first instance.
Confirmed in KDE 4.1 Beta1/Kubuntu.
*** This bug has been marked as a duplicate of bug 124482 ***