A few weeks back, I applied the latest updates to my Ubuntu 12.10 (32-bit) running Gnome-Shell, which happens to be a virtual machine running under VMware Player 5.0 on Windows 7. After the updates and a subsequent reboot, I didn't get the login screen. I could CTRL-ALT F1 to a text console, and I tried booting from the previous kernel, but no luck.
I discovered that if I disabled 3D acceleration in the VM settings, then the login screen would appear just fine. However, this isn't acceptable since the screen updates are just too slow.
I did a fresh install and after updating the same problem occurred. The same problem does not happen on 12.04 LTS. Not yet anyway.
I wrote a blog post describing the problem, and luckily someone left a comment that, although didn't directly fix my problem, helped me come up with a fix. Here's a link to my post: http://www.itadmintools.com/2013/03/gnome-shell-just-broke-on-ubuntu-1210.html
The problem appears to be a bug in the 9.0.2 version of the mesa 3D drivers. 9.0.3 fixes the problem, and they are located in the proposed repository.
P.S. I would have written this in the existing question (Ubuntu 12.10 no login screen after update (vmware workstation 9)) but it was locked and I don't have the rating to unlock it.