The just committed version 0.3 of the Plugin Properties/Templates of categories (serendipity_event_categorytemplates) now contains support to password-protect single categories.
The authentication is currently based on HTTP auth. Internal routines may get added later, but for the time being this allows for simple protection of entries set within categories. Usually you can already hide entries from special users (and groups), but those entries can only be seen when a user is logged in to Serendipity and it has to be set per-entry time and again.
With the new password protection you can effectively hide ALL entries in a special category from those users. Once they enter the category view for the protected category, the need to enter the right password (username does not matter) and then the users will see the entries of that category.
Have fun with the new plugin, which should be available in CVS and Spartacus within the next 24 hours. NOTE that this is a plugin for Serendipity 0.9! You will need to have the latest snapshot created tomorrow - on earlier 0.9 versions the calendar will no longer show entries once the plugin is activated!