Switching from Unity Desktop to GNOME3 fallback session in Ubuntu 11.10 (Oneiric Ocelot)
Installing the GNOME3 fallback session related packages and dependencies
First, you ensure already enabled with the universe and multiverse repositories.
Default the universe and multiverse repositoryes is not enabled. If you would like enabling this repositoryes, launch the software-properties-gtk preference tool, and enable this repositoryes.
You are possible to launch software-properties-gtk preference tool for example with sudo software-properties-gtk command in gnome-terminal.
If the universe and multiverse repositoryes is enabled, you need install gnome-session-fallback package. You possible using the Software Center or Synaptic this purpose, or possible using in terminal with following command:
sudo apt-get install gnome-session-fallback

Changing the desktop session from Unity to GNOME3 fallback session
Because LightDm login screen is not accessible for Orca Screen Reader, you manual need changing the desktop session. The simplest way is following this task:
1. Open the terminal with CTRL+ALT+T keystroke.
2. Run sudo gedit /var/lib/AccountsService/users/yourusername command to open the actual used session related configuration file.
If for example your username is john, the proper path is following:
/var/lib/AccountsService/users/jon
The opened file default containing similar lines:

[User]
XSession=ubuntu-2d

If you would like logging in next time with GNOME3 fallback session, you need replacing for example the XSession=ubuntu-2d line with XSession=gnome-fallback line. Save the file, and reboot the system.

Final customizations
When you logged again your system, you already have in the GNOME3 fallback session. Following part containing optional modification suggestion to prowide the new desktop session with better A11y experience:
Fixing "system menue" inaccessibility related problem
Unfortunately GNOME3 developers removed with the old "System" menue in the fallback session. Unfortunately, this modification are resulted visualy impaired users unable to access the importanter functions for example with the actual username awailable submenue (unable to change actual user status, launch system settings, log out, switch user, or shutdown the system.
Now, have only one known workaround to prewent this problem:
1. Press Control+Alt+TAB keystroke, and go to the top extended panel.
2. Delete this panel. Press SHIFT+F10 keystroke, and choose the "Delete this panel".
3. Go to bottom extended panel, and press SHIFT+F10 keystroke. Choose the create new panel menu item.
4. In the empty panel, you need add the "main menu" applet, and optional the "Clock", "Notification Area" applets. This last two applet is need only if you are not would like using GNOME3 ported Indicator applets development version.
This modifications are resulting you an one column menue system. If you are press ALT+F1 keystroke, you are possible accessing all importanter menu items. You will be see your username realted submenue, possible log out, switch user, or shutdown the system with keyboard usage.

Using indicators under GNOME3 fallback session
Unfortunately GNOME3 Indicator Applets porting related works is not finished before Ubuntu 11.10 development cicle.
GNOME3 ported indicator applets is already awailable with an external PPA repository. If you would like using the Ubuntu Natty release awailable indicators under GNOME3 fallback session, you need doing following steps:
1. Open the Terminal with CTRL+ALT+T keystroke.
2. Run sudo add-apt-repository ppa:jconti/gnome3 command.
3. Run sudo apt-get update, and sudo apt-get install indicator-applet-complete command.
4. Go to for example with top extended panel the CTRL+ALT+TAB keystroke. Press Shift+F10 keystroke, and choose the "Add the panel..." menu item.
If you would like using all indicators with a single applet, you need add the "Complete indicators" applet with your panel.
After this, when you closing the add panel applet dialog, if you press SUPER+S keystroke, you possible accessing all oldest time awailable indicators.