You don't need the Presenter.Presentations property. You are very rarely, if ever, going to have to try to load the presentations from the presenter, and when you do, you'll want to make this with paging,sorting,etc.
De-normalizing the Presenter in the presentation is good. You won't need to update the presenter name ever.
For example, I used to go mainly as Ayende Rahien, now I go as Oren Eini. Old presentations don't need to change, though.