A lesson

When I tried to do a parallel installation of KDE3 and KDE4, huge chunks of KDE4 started when I booted up my KDE3 desktop, wrote to the same .kde directory, upgraded chunks of it to KDE4, and terminally confused the KDE3 apps. This appears to have been because of KDE4's use of XDG autostart directories, which get used even if you're starting KDE3, unless you point the system somewhere else, in which case all your *non*-KDE programs that make use of XDG autostart are suddenly not started anymore.