Thunderbird 3.1.1 Ubuntu PPA Repository [Maverick, Lucid, Karmic, Jaunty and Hardy]
You could easily install Thunderbird 3.1 in Ubuntu using Ubuntuzilla, but that was only available for 32bit users.
A PPA repository is now available which provides the latest Thunderbird 3.1.1 (final, not daily builds) for Ubuntu Lucid - both 32bit and 64bit.
To add the PPA and install Thunderbird 3.1.1 in Ubuntu Lucid, simply paste the following commands in a terminal:
sudo add-apt-repository ppa:ricotz/ppa && sudo apt-get update
sudo apt-get install thunderbird
Update: I've replaced the original PPA in this post with a new PPA which provides Thunderbird 3.1.1 packages for Ubuntu Maverick (10.10), Lucid (10.04), Karmic (9.10), Jaunty (9.04) and Hardy (8.04)! The initial PPA only had packages for Ubuntu Lucid and only for Thunderbird 3.1 (not 3.1.1).