Alan Mendelevich on AdDuplex has some interesting discussion on the merit of the "trial" functionality in the Windows Phone Marketplace, arguing that the trial functionality is as essential to the experience as the metro design language. users expect to see trial version of applications in the marketplace, and those apps which ignore it are missing out on a lot of sales and potential points of customer engagement.

The same way users expect non-game Windows Phone apps to be Metro, they expect paid apps to have trials. Installation of an app is supposed to always be free. Purchase decision comes later. Even for 99 cent apps. It’s not about the amount of money, it’s about workflow.

It's actually a tough one to answer, because every application is different and it is impossible to have a cookie cutter answer. Right now, I think that to create an income stream from applications, developers can use ads or get users to buy the full app. Getting the balance in a trial version is tricky, and you get one shot at finding the sweet spot with each user.

And it's likely to change again when in-app payment becomes available to all. Microsoft and Glu will be watching the income coming in from Xbox Live title Bug Village very closely to see how the coin-purchasing system in that title goes. The ability to have purchases made from inside an application is another option for developers that gives them more confidence against piracy and end users 'not paying' for their work.

What we need are the success stories, the devlopers earning money, the ways they are doing it and concrete examples for others to see.

To paraphrase Hollywood, show the developers the money.