Missing Sync for hiptop enables synchronization of iCal events and To Do's list and Mac OS X Address Book contacts with the hiptop desktop interface and hiptop handheld (including the Sidekick). Share contacts, calendar events and to do items across multiple devices using iSync.

Markspace already makes a version of Missing Sync for the Sony Clié, now they are working on a version for the Sidekick. Website says coming soon...