First, stop plasma with
`kquitapp plasma`. Then open up
`kde4-config --localprefix`/share/config/plasma-appletsrcand put this somewhere in there:
Start Plasma again and voila.
If you're looking for a nice easy job to slide into Plasma or KDE development with, we need a configuration dialog done up for that. Two checkboxes and add it to the config dialog in Applet::createConfigurationInterface(KConfigDialog *parent) in kdebase/workspace/plasma/applets/systemtray/ui/applet.cpp. Easy peasy, and you'll make 4.3 that much nicer.
Update: A patch for this has already shown up on the plasma devel list. Nice..
Also, if you'd like to try your hand at some completely different way to show notifications or jobs in Plasma (or both!), the system tray uses the applicationsjobs and notifications DataEngines and so can you. Hook into either or both of those engines and all you have to care about is the presentation.