Microsoft did release a knowledge base article on which scenario's are supported and not supported with respect to SharePoint site definitions: http://www.kbalertz.com/Feedback_898631.aspx
One of the things that makes me really sad is the following statement:
"Microsoft does not support modifying a custom site definition or a custom area definition after you create a new site or a new portal area by using that site definition or area definition. Additionally, Microsoft does not support modifying the .xml files or the .aspx files in the custom site definition or in the custom area definition after you deploy the custom site definition or the custom area definition."
Besides ghosting I thought that exactly this point was the powerful thing about site definitions!! Back to the simple site templates... if you may not make any modifications afterwards so all instances of your custom site definition instantly reflect those changes, site definitions are useless!!!