Wow, I didn't see this one coming (though it's very welcome news):
...future versions of Eudora? will be based upon the same technology platform as the open source Mozilla Thunderbird? email program. Future versions of Eudora will be free and open source, while retaining Eudora's uniquely rich feature set and productivity enhancements. QUALCOMM and Mozilla will each participate in, and continue to foster development communities based around the open source Mozilla project, with a view to enhancing the capabilities and ease of use of both Eudora and Thunderbird.
Qualcomm press release:
http://www.qualcomm.com/press/releases/2006/061011_project_collaboration_mozilla.html
Also worth checking out is the Penelope project page, which is the official Mozilla project that will be dedicated to this effort:
http://wiki.mozilla.org/Penelope