After upgrading form iOS 5 beta to the final release, I cannot create any new reminders.
The issue seems to be caused by the fact that there is no reminder lists to which to add a new reminder, but I can't find any way to add a new list to the app.
In the screenshot, you can see I have the "Completed" list of tasks, that I made with one of the earlier betas of iOS 5, but there are no other lists.
The system's language is set to Italian, I hope that the screenshot is clear enough to show the situation.
Best Answer
On the iPad Reminders app, there is no + button when viewing Completed tasks.
What happens when you click on Data? Can you then add items?
Edit
It looks like others are having the same problem. Does this reply work for you: