True. You'll be unable to unlock any Windows Phone 7 devices until you've paid to join the program and your identity has been verified.
You need to submit three completely opaque .png images at 200 × 200, 173 × 173, and 99 × 99 pixels.
If you don't test your application under a number of different theme configurations, you're likely to fail testing because you might, for example, end up with white text on a white background. However, if you design your application to always have its own theme (and be legible under all theme combinations), you can happily ignore the user's selected theme.
Never use a time-based trial mode because the user can always uninstall and then reinstall your application to reset the timer.
The app's icon and title are the first things potential customers see of your application on the Marketplace.
You need to proactively seek reviews from website authors if you want to reach as many potential customers as possible.