Watch this video at Victor Gaudioso's Silverlight Blog
So there you have it a way to make a Sliding Panel using one ToggleButton ALL in Blend, using a custom ToggleButton with custom Checked and Unchecked Visual States, Storyboards and ControlStoryboard Actions.
The video is about 20 minutes but most of that was me trying to figure out what Storyboard needed to be hooked up to the Checked vs Unchecked States. Watching me make mistakes will help you to avoid them yourself, or so I hope.