I updated from 10.10 to 11.04 to discover my desktop icons flashing intermittently over the desktop background, with no Unity or other menus or panels.
The solution was to go to recovery mode, drop to root shell, and use (from memory):
mv /root/xorg.conf.new /etc/X11/xorg.conf
to setup my graphics card. At this point, X told me my graphics card wasn't up to running Unity and put me into Classic mode. Tada! Usable system. However, in the mean time in an attempt to solve my problem I've spent a lot of time on this and performed a complete re-installation.
I'm the only user of my computer and I had the system set to automatic login. It seems that if I'd had the login screen turned on I could have easily selected "classic" at this point and solved the problem. Unfortunately no such option or warning was given at any stage in the installation process. Interestingly, the Live CD ran just fine, defaulting to Classic mode without giving an explanation.
So my point is this - wouldn't it be nice if the update or the installation CD recognised that your graphics weren't up to scratch, told you, and turned Classic mode on for you automagically? Is this the place to suggest such things?
lspci | grep VGA
01:06.0 VGA compatible controller: nVidia Corporation NV34GL [Quadro NVS 280 PCI] (rev a1)