With Ubuntu 13.10...

gnome-session-fallback 

has been replaced with:

gnome-session-flashback

Gnome Flashback is a session for Gnome 3 which was initially called "Gnome Fallback", and shipped as a stand-alone session in Debian and Ubuntu.

It provides a similar user experience to the Gnome 2.x series sessions.

It consists of the following components:
  • GnomePanel
  • Metacity
  • GnomeApplets
The first release was on 2013-09-25, included in gnome-panel 3.8.0.

This is the first release that ships a session descriptor and wrapper script for the "Gnome Flashback" session. 

This is similar to the standalone fallback session previously shipped in Debian / Ubuntu. 

As integration with the rest of the Gnome stack has suffered somewhat, this is mainly aimed at distributions 
that still want to support a metacity / gnome-panel based desktop (e.g. for LTSP installations). 

Some integration work may be necessary - for example, nautilus 3.8 no longer draws the desktop background icons.

Brian