Introduction to the Model/View/ViewModel Pattern for Building WPF Apps

Model/View/ViewModel is a variation of Model/View/Controller (MVC) that is tailored for modern UI development platforms where the View is the responsibility of a designer rather than a classic developer. The designer is generally a more graphical, artistic focused person, and does less classic coding than a traditional developer.. The design is almost always done in a declarative form like HTML or XAML, and very often using a WYSIWYG tool such as Dreamweaver, Flash or Sparkle. In short, the UI part of the application is being developed using different tools, languages and by a different person than is the business logic or data backend. Model/View/ViewModel is thus a refinement of MVC that evolves it from its Smalltalk origins where the entire application was built using one environment and language, into the very familiar modern environment of Web and now Avalon development.

Read more at John Gossman's Avalon Blog.

Update: Some additional details here.