GNOME Shell is now available in the official Ubuntu 11.10 Oneiric Ocelot repositories, along with most of the GNOME 3 applications (Gedit 3, Nautilus 3 Evolution and so on; some of them are available for quite some time - like Nautilus, while some were uploaded recently - like Gedit or Evolution).
Note for those who are not up to date with everything that's going on in Ubuntu: GNOME Shell is not and won't be installed by default in Ubuntu 11.10 Oneiric Ocelot. However, Ubuntu 11.10 will use GNOME 3 with Unity instead of GNOME Shell.
GNOME Shell 3 (3.0.1) was uploaded about three days ago and the GNOME 3 session finally arrived yesterday. Here's a screenshot featuring GNOME Shell running in Ubuntu 11.10 Oneiric Ocelot:
(No, that's not the Ubuntu 11.10 Oneiric Ocelot default theme; I've used a different theme because Ambiance doesn't support GTK3 for now as you will see in some of the screenshots in this post)
After these updates, a "GNOME classic" session is available in the login screen, but it will probably be removed soon (because according to Mark Shuttleworth, there won't be a classic GNOME session in Ubuntu 11.10 - Unity 2D will be used as the fallback mode instead):
This is how the "GNOME classic" session looks like:
Here's an Ubuntu 11.10 Oneiric Ocelot Unity screenshot:
Another Unity screenshot, this time using a theme that supports GTK3:
And here's Unity 2D too:
Later update: AppMenu (Global Menu) with GTK3 support was uploaded a few minutes ago (the package isn't available as an update yet - it's still waiting in the "new" queue):
Important: Ubuntu 11.10 Oneiric Ocelot is currently in very early development stages (it didn't even reach alpha 1!) so a lot of things will change. Also, the theme looks broken because Ambiance (the default Ubuntu theme) hasn't been ported GTK3 yet.
Thanks to Dmitry Shachnev for the tip!