Once you've submitted your application, Microsoft will submit it to the testing and certification process, whereby Microsoft engineers run your XAP and try to find bugs. If your application requires users to create an account or log in, you need to provide Microsoft's testers with some usable credentials. If your application passes testing, you get an email saying that it's ready to be placed on the Marketplace, which takes a few days. If your application fails testing, you get an email explaining why, along with some suggestions about how to improve your application. Once you've applied these suggestions, you can simply resubmit the app to Microsoft. This process may seem overbearing, but it's a critical way to ensure that end users have a good experience with each Marketplace app.
You should thoroughly familiarize yourself with Microsoft's “Windows Phone 7 Application Certification Requirements” document, available at http://go.microsoft.com/?linkid=9730558. This document lists all the official reasons that an application may fail testing, although Microsoft's testers are allowed to use some discretion. The following are some common mistakes that can prevent an application from passing testing: