Today we published a code sample that implements a List Schema Designer for Visual Studio 2010 SharePoint projects. This is available as sample code that you can reference for your own designers or you can build and use yourself. You can get the version for Visual Studio 2010 beta here.
The extension adds an “Open in Designer” right menu command to list schema project items. Here’s what the designer looks like.
Please send me feedback on the designer. We will do some more work on this and publish an update for the release of Visual Studio 2010.