The iPhone as well as Android software (as far as I know) do not know a private mark. Rather it is possible to set up several calendars. I learned, that I can subscribe to certain calendars online - for instance to show the week of the year or to show sunrise and sunset.
So what I did now is the following:
- I created another calendar in my Outlook (on the Exchange server), named "Private", to which my colleagues do not have access
- My Outlook 2010 allows me to show the "public" calendar and the private calendar as an overlay, so I see all entries in one window
- my iPhone syncs perfectly with both calendars - so whenever I need to create a private entry I just choose it to go to the Exchange calendar named "private", whereas a public entry goes to the regular Exchange calendar.
In this way I am able to work fine with my iPhone's calendar system. Just a bit time needed to get used to it! :-) So, no need for any add-on! In any case, all googleing only led to the conclusion that marking a calendar entry as private is not possible with the iCal, nor is there any third party software which would offer that function (not even the well known pocket informant nor other similar software).