I'm using KDE 4.3 from latest Debian sid packages, I'm running sidux 2009-02.

Ksysguard displays my processes, their cpu and memory usage etc. correctly, but this isn't the case with general cpu/memory usage and the network monitor. All of these fail to give any information, and if I right-click on, for example, the memory usage graph, select properties and go to the sensors tab, it displays "Error" for both "sensors" (mem/physical/application and /mem/swap/used).

Does anyone know what I can do to get it working?

Try with a new user account, just to rule out if it's a misbehaving configuration in your /home.

I created a new account, but there I've got exactly the same problem.

If you open KSysGuard in Konsole, does it output any helpful debugging / error messages?

Nope, there's no output at all.

Do the panels in KInfoCenter function?

Yes, the memory usage for instance is displayed correctly. Weird...

When you start KSysGuard are any messages from "ksysguardd" printed to the file ~/.xsession-errors?

I solved the problem by - guess what - installing ksysguardd (Honestly, I really don't know why it wasn't installed, maybe it got lost somehow in the KDE 4.2->4.3 transition).

Well, sort of solved: it gives me correct values for memory and swap usage, but the cpu and network graphs don't even have a sensor connected to them, and I can neither edit them nor delete the worksheet they are on. I created my own worksheet and this works - if I could just get rid of the "built-in" system load worksheet...
Edit: After deleting the content of ~/.kde/share/apps/ksysguard everything works correctly

Thank you very much for your effort, I'm sorry that I didn't do the "is everything installed?" check before.

Edit: I posted the solution on http://sidux.com/PNphpBB2-viewtopic-t-17690.html