David Hill (Architect on the Microsoft Patterns & Practices team) has been working on a sample application that shows how you can use Prism and .Net RIA Services together. You can check out his blog post at: http://blogs.msdn.com/dphill/archive/2009/08/17/prism-and-net-ria-services.aspx
In the post he walks you through a sample application and shows how you can structure a Prism & .NET RIA Services solution that uses modules as a way to compose applications from both a services and a UI perspective. You can download the source code for the sample application here.
For additional background on PRISM, you may want to check out Shawn Wildermuth’s MSDN article: Composite Web Apps With Prism http://msdn.microsoft.com/en-us/magazine/dd943055.aspx