Release notes here: http://www.mozilla.org/projects/thunderbird/release-notes.html
The latest snapshot in portage is in fact 0.1. They haven't released a source tarball, so I had to roll it from CVS on the same date they released that version. Rest assured, they are equivalent. I will add an 0.1 ebuild as soon as they consider adding a source tarball important enough - it wouldn't help to get some more people bugging them about it, too. ;)