I've heard good things about Balsamic Mockups. Not specific to Silverlight however, and doesn't produce code, but is pretty good for UI mockups which is really the key feature IMHO.

+++ Rick ---

>We're looking for a decent prototyping tool for upcoming Silverlight development.
>
>It appears that the really cool tool is Expression Sketchflow. Looking at a couple of videos on it, we'd jump on it in a second if it could be had at a reaonable price.
>
>We have MSDN premium subscriptions, and I have my Certified Partner subscription - naturally none include Expression 4 Ultimate which is the only Expression version that includes Sketchflow. It appears our only choices for getting Sketchflow are upgrading our MSDN subscriptions to Ultimate (about $8000 UPGRADE price each per year), or buying seperate Expression Ultimate versions for $600 each or upgrading (possibly - not sure that works) for $350 each.
>
>Since we're not going to bend over for Microsoft, are there good alternatives for prototyping a silverlight site?