A new version of Thunderbird was released a couple days ago (November 14th). Here are the details from the Release Notes:

What’s New in Thunderbird 2.0.0.9

Release Date:

November 14, 2007

Security Update:

The following security issues have been fixed.

Earlier Changes:

For information about previous changes, please see the Thunderbird 2.0.0.6 Release Notes

Thunderbird 2 Features:

For an overview, please see Thunderbird 2 Features.

The Rumbling Edge has a more detailed list of notable bug fixes.

More info at Mozilla Developer Centre: Thunderbird 2.0.0.9 security and stability release now available