Nice question indeed - and winner of the current promotion. Well done.
I was wondering if anyone in the thread would like to create a wiki article covering in more detail the ideas and concepts here? I think "How do I create a trial app in Windows phone" which explores the difficulty of getting unique identifiers, server vs local storage, limiting on functionality rather than uses, in app purchase/advertising could make a very good story.
As a side note, I wonder if the in-app purchase could somehow be used here - ie is it possible to create a consumable (20 free goes) that can only be purchased once by a particular user?