Today I connected a second monitor to my computer. I set it up using nVidia's control panel, and all was working quite well, so I figured it won't be a problem to try Xinerama, just to see the difference between that and twinview. After enabling Xinerama and restarting the X session, I saw that I was logged into a Unity 2d session. I thought it was a problem with Xinerama, so I switched back to twinview, but it still logged me into Unity 2d. I tried disconnecting the second monitor, no luck: still Unity 2d. I tried changing GPU drivers and installing drivers from a separate ppa, and still I was logged into Unity 2d. Up until this point, I didn't have any problem logging into Unity 3d. It only happened after I tried using Xinerama.
I should note that I was doing all this while updates were going on in the background, so it could be something related to that, though I can't imagine what (I tried booting with another kernel, but no luck).
So what exactly happened? Did changing the mode to Xinerama triggered some other changes that I'm not aware of? Did these updates cause a certain malfunction in the driver? Is it something else?