Houman, we've let Apple know that we'd like the ability to offer time-limited trials of our iOS products, but we're dependent on them to make it possible; hasn't happened yet, I'm afraid.
That said, we do offer a
30-day moneyback guarantee on all our products, including the iOS ones. If the app doesn't work for you, we should bear that cost rather than you; we don't want to profit from selling apps folks don't need. :-)