Question: I'm having issues since yesterday after an upgrade. GNOME session got messed up somehow and now i cannot install further upgrades nor new apps. Here is what i got from terminal after doing the recommended `apt-get install -f`;

```
(Reading database ... 166876 files and directories currently installed.)
Preparing to replace gnome-session 3.0.1-0ubuntu1~build2 (using .../gnome-session_3.0.2-0ubuntu3~natty1_all.deb) ...

Unpacking replacement gnome-session ...
dpkg: error processing /var/cache/apt/archives/gnome-session_3.0.2-0ubuntu3~natty1_all.deb (--unpack):

 trying to overwrite '/usr/share/xsessions/gnome-shell.desktop', which is also in package gnome-shell 3.0.1-0ubuntu1~build1

Errors were encountered while processing:
 /var/cache/apt/archives/gnome-session_3.0.2-0ubuntu3~natty1_all.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
```

In `var/log/apt/history.log`, it says:

```
Start-Date: 2011-05-29  17:56:16
Commandline: apt-get -f install
Upgrade: gnome-session:amd64 (3.0.1-0ubuntu1~build2, 3.0.2-0ubuntu3~natty1)
Error: Sub-process /usr/bin/dpkg returned an error code (1)
End-Date: 2011-05-29  17:56:21
```

No idea what all this means.

Answer: 
I disabled GNOME 3 PPA, ran the upgrade, let it use the Ubuntu's original `gnome-session`, and then enabled back GNOME 3 PPA and ran the upgrade again. This time there were no dependency errors.
This sounds like a case of broken packaging in the Gnome 3 packages you're using: The new version of `gnome-session` contains the file `/usr/share/xsessions/gnome-shell.desktop`, which is also owned by the current version of the `gnome-shell` package you've got installed. The error is because of this conflict.

Perhaps check if there is a new version of the `gnome-shell` package available in the PPA you're using?