When Joomla! 1.5.0 stable was released, we received many requests for JCal to be natively compatible. Thus, we decided a few weeks ago to rip JCal apart and rebuild it using the MVC framework. We chose JCal Pro® 1.5.3 Stable as our codebase since this version is widely used with relatively few bugs.
We also decided to put JCal Client 1.6.x on the backburner while we completed this task. We expect a beta of JCal Pro® MVC (Joomla! 1.5 native) by the end of the month. Following a bug-fixing session, we will then resume development of JCal Client 1.6.x.
Thank-you for your patience and continued support.