AW Systems has just released an open source tool called SLAM (SharePoint List Association Manager). Check it out now on CodePlex at http://www.codeplex.com/SLAM.
SLAM lets us do just that by allowing developers to describe complex relationships between lists and content types in a way not previously possible in SharePoint (at least not without a great deal of time and effort). With SLAM, these associations are managed automatically. Developers can then concentrate on building web parts and Custom Field Types that draw on those relationships using familiar and straightforward SQL queries.