i3 started from lightdm properly, but autostart did not work. The line
"X-LightDM-DesktopName=i3" seems to be responsible to allow autostart.
Tested on a clean install of Ubuntu 14.04.
With this file, you can use the following command to run i3 in GNOME:
gconftool-2 -s /desktop/gnome/session/required_components/windowmanager i3 --type string
Holger Langenau