Well another new world has opened in the universe of Microsoft Dynamics CRM and a number of long time Dynamics CRM Guru's have questions.
To get you started on what can be extended in the new forms take a look at this great little blog post just released on MSDN
http://blogs.msdn.com/b/crm/archive/2013/03/11/customizations-in-december-2012-service-update.aspx