Currently there is a gap in the application that prevents customiziers to create sub-grids within a Form view that retrieves data based on a lookup field. For instance, I want a view where I can see all of Accounts where the Primary Contact on the opened Account is a Primary Contact. For instance, Jane Doe is a Primary Contact for Account 1, Account 4, and Account 6. When opening Account 1, you have a subgrid that shows Jane Doe is the Primary Contact for Account 4 and Account 6 as well. This is documented here:
http://www.magnetismsolutions.com/blog/paulnieuwelaar/2013/01/24/filter-sub-grid-dynamics-crm-2011-rollup-12-(polaris)

Currently the only way to achieve this is by using the DOM but as we all know, utilizing the DOM is unsupported. Also in CRM 2013, the workaround suggested above will not display data in the columns of link-entities defined in the FetchXML.

This has been requested by many customers and there are many blogs or other articles that try and workaround this limitation in an unsupported manner.