Jamie checked in thunderbird support to Tracker today. How do I know ? Because I subscribe to svn-commits-list and my gmail filter separated him out when he was adding the beagle thunderbird backend (probably with some modifications, haven not checked the changeset) to tracker.
This is looking pretty cool. I also found out Tracker-0.6 was released with some extremely useful features and a damn good UI. The UI groups the documents in an intuitive way, shows more context information from matches, allows you to do tagging inplace (and has gradients, uh-on). The core daemon (is that what it is called ?) can scan evolution mails, and pidgin logs. Now it can also do index thunderbird mails. It lists XMP metadata support too - which is nice and something I never managed to finish in beagle.
I am pretty sure people will find it useful. I could not resist myself to admire it and share it with others. Well, I was told that tracker is set to become the default desktop search engine in ubuntu, so a lot of unhappy souls are about to become happy soon.
For me, I will continue using beagle. Why ? Because, it does what I need it to do. And if it does not do something, just remember it is written in C# :). Meanwhile, you make your own decision.