In my experience, the integrator or "devsigner" role really needs to be involved in this process unless everyone on the (small) team are able to perform this role. This is a very rare circumstance. Usually you will find that developers are very good at developing but aren't so great with design/usability and designers are great with aesthetics/usability but don't want to or are not educated enough to code. Having someone that can crossover into both worlds and "speak the language" is very important.
The integrator needs to coordinate the controls that are being developed with the design assets that are being created by the designers. In our current project, we have 6 active developers and 2 designers from an outside shop. I am the integrator for this project and I spend most of my day in Expression Blend. The developers work primarily in VS creating controls that meet our product spec and the design shop is designing what the end product will look like. The designers are working in Illustrator. My job is to take the Illustrator files and create control styles from them and then apply them to the controls developed by our development team. As we move towards Blend 3 with native support for PSD and AI files, this task becomes much easier.
It is very helpful to create the "look" for your application in a separate solution from the main trunk of the application and then merge your ResourceDictionaries into the main app later. You can get the look and feel correct without getting too caught up in what could still be incomplete controls.
