Hi barry - I'm the admin, but the multi-site is for an academic department, so it's tricky. I've notified the other admins that I'll be playing around, and asked when would be a good time to disable plugins on a rotating basis to try and isolate which one is causing the problem.
It's not a theme issue - I've tried three other themes - one is TwentyEleven - and same thing.
Could this be a residual problem with the posttype? I had tried a different plugin for events, but ripped it out. Could there be something still left in the tables for post types that might be causing a hiccup?
I'll let you know what this turns up.