An article I worked on has recently been published on MSDN. The article, Creating Custom SharePoint Server 2010 Field Types and Field Controls with Silverlight 3, demonstrates how to create a custom SharePoint field type and field control that leverages Silverlight 3 as the editing and display experience.
While the article is written for SharePoint 2010, it does also apply to SharePoint 2007. The only real difference is that you don't have access to the rich Visual Studio 2010 SharePoint Development Tools and that your SharePoint 2007 server is not configured OOTB to support Silverlight 3. You'll have a bit of extra work to get that done.
SharePoint Foundation 2010 offers developers the capability to create custom field types and controls when a custom data storage, edit or display experience is needed. When combined with Silverlight 3, developers can create rich and engaging experiences for users. This article covers the details on creating a Silverlight 3 application, deploying and leveraging it in a custom field type and field control sample for SharePoint Foundation 2010 and SharePoint Server 2010 Publishing sites.
You can also download the code sample associated with this article from the Critical Path Training members download section (login required... if you don't have one, just create a free account to get it).