Scott MacGregor writes: "I'm excited to announce that our first Thunderbird 1.0 Release Candidate is now available for testing. 1.0RC1 includes lots of bug fixes and improvements for features like saved search folders, the RSS reader, mail migration, and message grouping. The default themes have both been updated with new improved artwork as well."
Scott's post to the Thunderbirds Builds forum about 1.0RC1 has more information. The release candidate can be downloaded from the 1.0rc directory on ftp.mozilla.org.