I tested something with your setup. Created an entity Car and 2 entities for the Honda and the Toyota that have the generalization Car and the newedit forms for those classes. Created a template grid for the entity Car, displaying the Car attribute (for the test just 1 Name) and a dataviewlist with entity Car and 2 form mappings. One for toyota and 1 for Honda. Added an Xpath [id = '[%CurrentObject%]'] to make sure that the car displayed in the template grid cell and the dataview list match. When creating an new Car with the standard New button I see the behaviour that you mentioned, only the loading Icon is shown. Which is correct as the created entity is of class car and this I didn't map as a form in the dataviewlist. When I add a second new button to the grid and make sure this button creates the Honda (e.g.) then the dataviewlist in the template grid will show the form that was mapped. After mapping the car_newEdit form for the datviewlist the grid will also display this form in the dataviewlist.
Server version used 2.5.4 tested in firefox 4.0.1
I hope this helps, if not please let me know.