I've seen this problem reported under several questions, but I haven't been able to resolve any of it so I thought I'd bring it all in under one umbrella.

I started a new job and was given a Dell Precision M6400 laptop with Nvidia Quadro FX 2700M graphics card. It had a previous version of Ubuntu on it, but nobody had any passwords for it so I wiped the drive and did a fresh install of 11.10 from scratch. I didn't do any updates during installation, preferring to do them after boot. Updates ran fine and the system works... except Unity is in 2D mode.

System Settings -> Additional Drivers shows that Nvidia-Current is active but not in use. System Settings -> System Info shows Graphics Driver Unknown, Experience Standard

Nvidia X Server Settings is installed and working, re-writing the xorg.conf did nothing.

/usr/lib/nux/unity_support_test -p

OpenGL vendor string: NVIDIA Corporation

OpenGL renderer string: Quadro FX 2700M/PCI/SSE2

OpenGL version string: 3.3.0 NVIDIA 285.05.09

Not software rendered: yes

Not blacklisted: yes

GLX fbconfig: yes

GLX texture from pixmap: yes

GL npot or rect textures: yes

GL vertex program: yes

GL fragment program: yes

GL vertex buffer object: yes

GL framebuffer object: yes

GL version is 1.4+: yes

Unity 3D supported: yes

One suggestion was to do a sudo apt-get --purge remove nvidia* and that resulted in a scrambled screen on boot and a non-bootable installation. Pressing the Delete key on boot allowed me to access the recovery console and do a sudo apt-get install nvidia-current, which brought me back to a working, bootable system.

Another suggestion was to edit /etc/default/grub and change the line reading "GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to read "GRUB_CMDLINE_LINUX_DEFAULT="quiet splash vmalloc=192MB" thus allocating more video RAM.

I did that, followed by a sudo update-grub and a re-boot. No change.

Created a brand new standard user and logged on with that account, no change.