I unpacked a new Mac, put it in Target Disk Mode, and copied the old Mac's drive over with Carbon Copy Cloner - but I forgot to tell CCC to erase the new drive first. That left the new drive with everything from the old machine (10.3.4) copied on top of the software preload of the new machine (10.3.2).
The Mac booted, but immediately launched the preload software installer (choose language, insert DVD 1 etc.)
Since I had copied the entire running system of the old machine, I figured all I needed to do was disable the preload installer. As it turns out, the preloaded system has a 0-byte file
/var/db/.xSelectNeeded
that triggers the installer. Deleting the file solved the problem, everything works fine.
Note that new systems (e. g. the new all-dual G5) seem to ship with a complete preload of 10.3.4 and no longer request the installation DVD. Instead, they bring up the new "transfer data from previous Mac" dialog. I don't know whether they still use the ".xSelectNeeded" file.
Obviously, skipping the preload is probably not a good idea if you haven't copied over a previous installation like I did - unless you know what you're doing. :-)

