CalTerm scratches my own itch. I wanted a low-bandwidth, standards-compliant (i.e. ical suppport) calendar app for use with my Sidekick/Hiptop mobile device. I live in the terminal, though, so when I get around to making it support editing/writing calendars, it may also replace Sunbird/Lightning for all my calendaring needs (except email integration at work).
You may find either the README or TODO file of interest.