Comment 2 for bug 707232

@Stéphane: in fact, it's not really the cause :)
The cause is that it tries to launch /usr/share/gnome-session/session/gnome.session by default, which is correct, but this one try to get acceleration and doesn't fallback because of a key change.

So, what I'll do for now:
- change the default to ubuntu
- fix the gnome.session file to the new syntax

Not sure that the ubuntu session will works for you as it's testing for acceleration, so it will probably fallback