Ubuntu 11.10 is falling back to Unity 2D. How to get back to Unity 3D?

Finally solved the problem here! Like @MichaelK said:

[...] Most likely the driver switched to another one and you lost opengl support [...] Well update information on your graphics hardware and driver as well.

So the way I found to reset things was to just purge and install xserver-xorg, after that everything went back to normal:

$ sudo apt-get purge xserver-xorg
$ sudo apt-get install xserver-xorg xserver-xorg-video-all
$ sudo reboot

Voila! Hope it helps.

ps: The only bad thing now is that every time I plug a secondary monitor on my computer (specially when showing Ubuntu to someone) I have to cross my fingers secretly. This driver switching behavior from Ubuntu is no good man ¬¬