I know people have asked about this feature for flash encoding as well.
In flash the actual poster frame is set in the player code. I've done this with JW (now Longtail) player.
What I'd like Episode to do is generate a series of poster frames automatically based on duration points such as beginning, middle end, or every x minutes, etc, or allow the user to select one for an encode otherwise one is having to go through each source by hand to select an export a frame. You can export an individual frame in Episode's preview.
If you upload video to YouTube or Vimeo they're batch encoding is generating poster frames based on duration/location.
As far as Windows Media is concerned I assume one might be able to link to a poster frame using Silverlight wrapper but there may be other ways as well.
I have told Telestream there's a demand of poster frame generation though.