According to the Plugin Compatibility list on the WordPres.org site, EventCalendar 3.1.1. RC1 shows as
compatible not quite compatible with WordPress 2.3. Good deal!
UPDATE: September 25, 2007…I’m going to stand by my initial assessment that EC 3.1.1 is at least a little broken with the WP 2.3 upgrade. I just can’t see how some of the advanced EC queries that access the Post2Cat table will not return an error. Perhaps most of the EC functionality is unaffected but…Now the Codex has been updated to note EC 3.1.1. will give 404 errors when you keep events separate.
UPDATE October 1, 2007: EC 3.1.1 RC2 is out to address the database schema changes in WordPress 2.3 and has backwards capability with WordPress 2.2.x and prior. Works like a champ on my 2.2.x install.