There are no proprietary restricted drivers with PPC. The good news is that it is working with your external monitor.
What you have to do now is get around the Apple display quirk that doesn't like to play nice with Linux.
You didn't mention if you just got a black screen, or low-res graphics with your internal display.
At any rate, because this is Ubuntu, the first thing I'd do is bypass the splash screen.
At the second-stage of the yaboot boot: prompt, hit TAB to stop the countdown. Then enter:
If that doesn't work, you could then try an additional parameter:
(Capital L for Linux btw...)
Linux nosplash video=ofonly
It could go beyond that however, requiring a custom /etc/X11/xorg.conf file to force the correct vertical and horizontal frequencies that your powerbook uses.
In addition, you may have to force the resolution, say 1024x768 mode as well.
Since I don't own a powerbook, perhaps another owner can help out. In the meantime, you can see some generic instructions for xorg.conf in this thread that may help even though it is not powerbook specific: