I am starting to work on a Silverlight 4 based web mapping project using latest ArcGIS Server API. The application will involve multiple map services (for many themes) with selection, queries, and reporting for each map service. I am starting to gather some ideas on how to make this application extensible so that any additional map services can be added later on. For this project, in addition to map services, I will be using Oracle for backend for non-spatial data access. I am not sure if this will impose restrictions while building WCF services considering the fact that it's not SQL Server.
I am looking for some feedback from folks who have implemented Silverlight based GIS solution and some of the factors that needs to be considered for a good/robust design. These would include: MVVM, logging, unit testing, security etc.