Ubuntu 11.10 Alpha 2 Brings Thunderbird, LightDM
Some of the key updates in this second development release of Ubuntu 11.10 is the Linux 3.0-rc5 kernel, GCC 4.6.1, Mozilla Firefox 5.0, Mozilla Thunderbird 5.0, and a Mesa 7.11 development snapshot. What's noteworthy about Thunderbird 5.0 is that it replaces Evolution as the default e-mail client in Ubuntu Linux.
Also found as part of the Ubuntu Desktop in Oneiric Alpha 2 is more GNOME 3.0 integration (including some GNOME 3.1.2 Alpha components), Deja Dup now provides backup capabilities, LightDM instead of GDM as the log-in manager, and many other changes.
The Oneiric Technical Overview Alpha 2 page has many more details about the Ubuntu 11.10 Alpha 2 snapshot. There's also Kate Stewart's mailing list announcement.
Ubuntu 11.10 final will be released this October.