In this post, David Anson demonstrates one WPF-specific fix for his SplitButton/MenuButton.
I've previously written that one of my ContextMenu test cases for the April '10 release of the Silverlight Toolkit was to implement a quick "split button" control for Silverlight using Button and ContextMenu. Though it wasn't my goal to build a general-purpose control for widespread use, there's been a lot of interest in SplitButton/MenuButton and I followed up with a few fixes for the Silverlight version and "official" support for WPF.