One of the interesting things about WPF apps, and Silverlight 2.0 apps for that matter, is the number of ways to go about constructing the app. A Designer type can put together an app inside of Blend and integrate design assets as they're built, but then the Designer is blocked waiting on controls or events from the Dev. A Dev with understanding of Blend could put together most of the project and handoff to a Designer Integrator to implement the design assets from the Creative team. Or a Dev could construct the app's architecture, lay down the controls built to support the necessary functionality, and let the Designer Integrator style them - this removes the Creative team from the workflow and is not very smart most of the time. The bottleneck on this platform is often times the Creative Designer since they're often blocked by the Information Architect who is trying to gauge the correct wireframes to suit a select piece of functionality all while working with the Technical dev, the Designer Integrator, and the PM to validate not only the concept's feasibility, but applicability inside of that space...repeat dozens of times for a given project. A challenge for a PM is to assure that all resources on the team fully understand which workflow is going to be used and sticking with it. Still, it's hard to measure progress linearly. The critical path is the Information Architect and his/her ability to swiftly move through bits of functionality. In a perfect world, all screens are wireframed and approved by the client prior to beginning this dev/designer workflow. This needs to be taken into account and planned for, but the real kicker is either getting the entire project team onboard from the inception of the project and keeping them there until the wireframes are all complete, which increases project costs, or having a well oiled creative group capable of making spark decisions on wireframes, decent comps that support the business need, and just enough umph to wow the customer and exploit WPF or Silverlight without forcing a design interaction. Often times, the Dev and the Interactive Designer (Integrator) are left to stumble building an app in hopes that the design assets being produced will hold true to the vision of the architecture being put in place and can be implemented. There's something to be said about measuring thrice and cutting once, but in the production world of software dev, customers rarely support the notion or even respect the ingredient of time; so we're often left stumbling through the wireframe definition phase in parallel with the early development or prototype.