I just reinstalled Ubuntu on one of my laptops, an HP Pavillion with Nvidia graphics. This was a fresh install replacing 10.10 with the new 12.04. I use an external monitor plugged in via VGA because the screen on the laptop is broken so badly that it cannot be read. All went fine during installation, but upon reboot the fresh install does not recognize the external monitor. Naturally I am able to log in by just hitting the Enter key and inputting my password and hitting enter again. I tried this, hoping it would see the external monitor after login. It did not. I can tell it logged in properly because the broken laptop screen changed colors. I'm sure I could edit a config file or something but this is difficult to do on a laptop without a working screen. As I write this, I am reinstalling with the "download updates" option selected in hopes that this may fix the issue. I would rather just be able to change monitors with a keyboard shortcut but I don't think this is possible. I did not have any such problems with the old 10.10 installation. Any help resolving this issue is appreciated.
I think you are on the right track with editing the xorg.conf file that you posted not long after this one. I suspect the reason you didn't have this issue in 10.10 was rooted in how Xorg auto-configs. xorg.conf is now only really necessary if you have special circumstances like a broken monitor. :-)