When developing applications it is always a good idea not to hardcode database connections, urls, server locations, etc... In .NET there's the Configuration stack that allows us to use the app.config xml file. Unfortunately this functionality doesn't exist in Silverlight as well as in the Windows Phone 7. However I always remember that there's an excellent set of Mobile Application Blocks that includes the Configuration block (I have been successfully using this block on all mobility projects I've ever been involved). It didn't take me too long to make port of this block's code to the WP7 platform. Most of the changes that I had to make where related to the usage of the non generic collections and the fact that we need to use the Application.GetResourceStream method instead of the file IO access. The best thing is that the usage of this block is very close to an existing .NET code which requires a few steps. First you'd need to create an implementation for your ConfigurationSection, ConfigurationElement, and ConfigurationElementCollection. Then you add app.config file to your project and make sure that it has the the Build Action set to "Content". The last step is to add the code that reads the values from this configuration file:
You can download the sample client that includes implementation of the ApplicationSettingSection as well as the port of the Mobile Configuration here.
P.S. This port of the Mobile Configuration block should work just fine in desktop version of the Silverlight.