YMMV I guess. I just hooked a new monitor up to my laptop running 12.04 last night. I had to manually install the ATI drivers, then change the settings in Catalyst. I was required to reboot after each change. Then, I had to change the display settings in system settings. It took about 2 hours to get right and was a huge pain.
I have a dual monitor set up at work on 12.04 and it works flawlessly & smoothly. It is absolutely hit-or-miss. There are driver issues on Ubuntu for dual monitors.
I use AMD on a desktop and a laptop and dual monitor works perfectly in Ubuntu 12.04 with both in various setups.