Hey, I had this problem with LinuxMint too, but not Ubuntu. I'm running Fedora 15 on an Asus U30JC, and I have a Lenovo monitor that I'd like to use as the primary monitor. I plug it in and it automatically extends the desktop onto the external monitor - this works fine. However, when I go into the display settings, and turn off the laptop monitor, the laptop monitor shuts off, but the external monitor just has a broken image. Most of the time it's just black (but on) with a flickering mouse pointer in the bottom left corner. Once it's shown broken up pieces of some open windows, but just flickering again so I can't do anything with it. After a few seconds, it reverts to previous settings and everything is fine again (desktop extended onto the external monitor). I've tried changing the settings on the external monitor (i.e. reduced resolution, etc.), but it does the same thing.
Does anyone have any suggestions?