At the recent Phoenix Comicon convention (May 23-26) JMS requested a special panel that he asked all fans to attend. In this panel, he paid tribute to Rick Biggs, Andreas Katsulas, Jeff Conaway and Michael O'Hare. In particular, JMS kept a promise he'd made to Michael O'Hare to tell the true reason why Michael left the show after the first season. Here's a link to that panel (chosen for sound quality rather than shooting angle):
http://www.youtube.com/watch?v=4yjOsaa7XL4
It was an amazing tribute and an important moment in Babylon 5 history.