FWIW my calendar has begun acting wonky since iOS5 as well (on my iPad2). Different than OP's issue, but still worth noting as my Calendar was rock-solid until the upgrade. For me, one of two current issues is that when I first open the calendar, while it opens to the correct month...that month is entirely blank of meetings/appointments. The dates/days are there, just nothing I've scheduled. Now here's the thing...all I have to do is shift to a month before or after (which have their appts just fine) and as soon as i return to the current month, all my meetings/appts now display. The other issue I'm experiencing is that setting up meetings/appts get flaky now and typically 'poof' (disappear) before I am able to complete the Save. Quite annoying.