Thunderbird 3.1 release candidate 2 is out - you can read the full release notes HERE.
Unfortunately for Ubuntu users, this version is not available in the Ubuntu Mozilla Daily PPA but if you're using the version from Mozilla's website, you should receive an update today. If not, you can install Thunderbird 3.1 RC2 from Mozilla by using these commands (for any Linux distribution):
wget -O - http://releases.mozilla.org/pub/mozilla.org/thunderbird/releases/3.1rc2/linux-i686/en-US/thunderbird-3.1rc2.tar.bz2 | tar xj -C ~
The above command is for EN-US language. For other languages, edit the link above with the download link for your language which you can find HERE.
If you used the package from the Ubuntu repositories, this step is not required, but if you've installed Thunderbird 3.0 or 3.1 from a PPA, you can migrate all your profiles to the new Thunderbird installation using the following command (make a backup first!):
cp -r ~/.thunderbird-3.*/* ~/.thunderbird/
Then to run it, simply double click the "thunderbird" (not "thunderbird-bin"!) file from the "thunderbird" folder in your home directory.
The advantage of using Thunderbird directly from Mozilla's website is that you'll receive the updates directly from Mozilla - so you'll get them a lot sooner. This means that next week when Thunderbird 3.1 final will be released, you'll receive an update in Thunderbird and you'll no longer need to wait for a PPA to get Thunderbird 3.1 for your Ubuntu version.
Also, the package from Mozilla is built for i686, not 386 so it should be better optimized for newer processors.