I realize this is an old post, but the issue still exists in 12.04. Actually not just Ubuntu, but any distro I have tried.
On my PowerMac G5 with Nvidia FX5200, the work around is to disable the (non-existent/phantom) TV output. No xorg.conf needed.
Depending on your monitor, you may also need to get more explicit. IE;
boot: live video=TV-1:d
boot: Linux video=TV-1:d
I attempted a "step by step" of how I got there in this linked thread.
boot live video=DVI-I-1:d video=DVI-I-2:1280x1024-24@75 video=TV-1:d