Comment 31 for bug 856810

Here is my experience with this issue:

I have a fresh install of 11.10 running since Alpha 2 - NEVER had ANY sorts of problems

I have another image running 11.04 without any problems. Today I decided that I would upgrade my 11.04 to 11.10 and since my 11.10 did not show any problems, I did it with confidence.

After reboot, I saw the "waiting for network" issue. From there, it booted to a blank screen.
Tried booting with the splash turned off in text mode and noticed dbus was not starting followed by the waiting for network issue. Still it booted in to blank screen.

Next tried to boot in to single user mode, noticed the network no longer lists eth0 in interfaces file.

tried booting with "init 5" from here with no success.

Then read about the lightdm issue and migration to /run, so I did as follows:
0) Start dbus manually
1) start "gdm" from single user mode
2) log in
3) desktop appears, unity appears, task bar appears
4) start ANY program, the icon shows in unity but the app window is not visible. Not a single window is visible ... even shutdown dialog
5) reboot

Repeat the above steps, but now select "ubuntu 2D" in step (2) above. Now its fine, can see app windows, interact with them etc. No network still.

Now I did
1) run dpkg-reconfigure lightdm like "Michael" said above
2) Moved the files from /var/run to /run and /var/lock to /run/lock and made symlinks (thanks Gabriel Thörnblad)
3) Cleaned out the dbus directory under /var/dbus (from Alexandre Devaud #17)
4) install VBOX addons (again ... to try to get network back)
5) reboot

Now it seems like everything is fine.

I suspect
upgrade script is not working properly and failing in migrating the file from /var/run & /var/lock to their new location. This is causing the dbus to fail and hence lightdm gets messed up and doesnt start which results in blank screen and no app windows.

Not sure why my ethernet related settings disappeared though.

Hope this helps others