Since the project appears not to have a mailing list of their own, so I figured I may as well ask here
It was effectively killed (or put into hiatus) when Roger Dingledine hired Nick Mathewson to work full time for him on tor a couple years ago. Mixminion was effectively Nick and Roger's baby. Roger jumped first to pursue Tor full time and then brought Nick over later. Since then nobody had picked up the torch. I spoke with Roger about this a couple years ago at Defcon and his take was basically he could do more good (and arguably has) with Tor than he ever could with Mixminion given the steep entry curve for normal users to Mixminion, the indifference of the layperson to anonymous email given the growing Web 2.0 post-email web forums and blogs, and a greater need for anonymous surfing, posting, and tcp streams than email. I have cc'ed Roger on this if he wants to add further information; I don't have Nicks email. -Peter