The WPF and Silverlight versions of SyntaxEditor share our next-generation codebase for the product. So everthing from lexer, parser, grammar, and IntelliPrompt will be the same between those versions. For instance, our .NET Languages Add-on shares the entire codebase between WPF and Silverlight, so when we make changes to the languages there, it's very simple to copy the changes to the other platform.
The WinForms version of SyntaxEditor uses our original object model so while there are a lot of feature areas in common, the implementations are very drastically different than in WPF/Silverlight. As we have ported feature areas over to WPF/Silverlight, we've focused hard on making everything extremely extensible and adding features/options along the way. Once we get all the same feature areas covered in WPF/Silverlight that are currently in WinForms (we are getting closer to that, and the major ones are already over) then we hope to eventually port the next gen object model back to WinForms so that all three platforms can share a codebase.
One a side note, if we do a WinRT XAML (for Windows 8 apps) port of SyntaxEditor, it would also share the next gen codebase.