Consonance now has a calendar UI for the scheduler tab of the main window. This is a significant milestone; up until now that feature has been very conspicuously absent. I had hoped to be be able to port some code from some other projects but ended up writing the entire widget from scratch. Consonance displayed a calendar event for the first time on 2007-12-30 at 16:33 (GMT). A screenshot is here and also attached to this post. This code is included as of r470.
Jan Schneider mentions support for WebDAV, CalDAV, and GroupDAV in her blog, here. GroupDAV support in Horde adds thousands, if nor more, compatible servers to the world. Also interesting is the mention of DIMP an AJAX-ified version of IMP. Great news for the Horde and GroupDAV projects.