Not yet...
It's a feature we 're all waiting for !!!
Bug 6460 - Enable shared calendar entries to be marked private
It's noramlly targetted for Version 5.0
For now, you can create another calendar to put your private events, and you don't share this calendar.
- People will see you're not available when they check your availibility
But :
- If you sync your calendar with a mobile device, only your main calendar will be sync'ed
- People who can see you calendar won't see anything in calendar view for your privates events (this can only be seen in the plaanig pane when they create an event)