The screensaver problem this page relates to was fixed in KDE 3.5.4. At time of writing 3.5.5 has been out a while. I recommend you update to the latest version of KDE rather than use the fix available here.
[K]Ubuntu users see http://kubuntu.org/announcements/kde-355.php for Dapper repos or just update to Edgy or whatever the latest release is by the time you read this.
In KDE 3.5.3 the screensavers are broken. They just don't kick in after the designated time. This is a known bug in the kdesktop package and has been fixed in the KDE source. For details see http://bugs.kde.org/show_bug.cgi?id=128610
This doesn't help people like me running 3.5.3 installed via packages provided by their distro though. Tired of having no screensaver I decided to attempt to get those fixes on to my system. Somewhat surprisingly I succeeded. I grabbed the latest source for kdebase, got the kdesktop stuff compiled, identified the file affected by the bug, copied it the relevant location and lo and behold the screensaver worked again. You can download the file (for 32bit and 64bit x86 systems) at the link below. If you have a something other than an x86 system or just don't trust that the binaries for whatever reason then I've done a guide on how to build the file N.B. USING THE GUIDE IS PROBABLY A BAD IDEA. I have left the link for historical interest or whatever. If just follow the guide as is you'll be getting snapshots for whatever the latest version of KDE is, i.e. not for 3.5.3. Will I update the guide? Nope. As I said up top, I recommend you upgrade KDE.
The location given for the file is correct for [K]Ubuntu 6.06. Check for the existence of the file in the location stated before copying the file there as it may be somewhere else on other distros. If it's not where I suggest use locate to find it. e.g.
$ locate libkdeinit_kdesktop.so
Don't just click on the link, do right-click and 'save as'. You will have to log out and log in again to make the new file take effect.
Put this in /usr/lib/ (make the permissions match the original file it replaces)
32bit x86 - libkdeinit_kdesktop.so filesize: 752k md5sum: 8b562c08e249a0f63b97a8ae88a652e7
64bit x86 - libkdeinit_kdesktop.so filesize: 883k md5sum: eda7030826a5806836b83b1303c2e0de (File courtesy of Joe Hart aka Lil_Eagle)
These files and instructions are provided as is. No warranty etc etc. This fix is not endorsed by the people who produce KDE. If you use it and your system gets borked or your hamster dies don't come moaning to me. Make a copy of the original file on your system first so you can reinstate it if need be.