My powerbook is now able to use the external VGA!
After hours of trial+error and reading the XFree86 radeon driver, I now finally managed to get the external DVI/VGA port of my Apple TiBook IV to display something useful. CloneMode didn't work for some strange reason, but I'm now running a multihead setup.
This means, that at the next conference I can give my presentation with just one single notebook, no need for second notebook, crossover cable and remote X display anymore. If that isn't good news...