How to sort mail.app conversations by subject rather than sender?
Ah!  I think I have actually found the answer.  In the Mail.app with Lion, it sorts by whether or not there is a Re: in the subject.  So all of those ticket emails up there, which start with the ticket tag, would thread correctly if they started with Re: [#tag] instead.  I am about to add Re: to my RT subject lines, to fix this problem for my own ticket traffic.