I'm the developer and the designer too. I think developers love blend, because it is very simple, and it is easy to create attractive interfaces (the greatest programming/design IDE i've ever seen). But i never met a silverlight designer, who is not a developer too. I think it will be happens in the future, that completely separate design and coding, but not now. Maybe in a new programmer generation :). I think its very important for a silverlight/wpf designer, to know, how this new binding concept works. Programmers knows it already, so it is much easier to do coding and design for yourself, than ask a designer to learn blend and the silverlight/wpf things, because they think it's coding...but blend is simply clicking...but you must learn it, and you must understand xaml hierarchy and syntax and everything...
So i think they must go to a training class, with developers, and learn the new things together. Than they can separate things. Designers are much lazier than developers, but they need learn new things too.
I think i'm lucky, i love design & coding too.