At Readify we focus on the Microsoft stack - WPF, Silverlight, and ASP.NET. The bulk of our projects are ASP.NET, some are Silverlight, and even less are WPF (though the WPF projects tend to be bigger in scope, so they're probably about even).
Usually the customer has decided on a technology stack before we arrive, and they've engaged us to help plan/design/build/ship the application. Sometimes it's too late to suggest technology changes, but other times we can influence the technology they decide to use.
When I'm faced with deciding on a technology, here is my workflow.
Read more: Paul Stovell