I propose to rename the "virtual" option to "inherit_data". The reasoning is quite simple: While it is necessary to explain what a "virtual form" is, the name "inherit_data" is self-explanatory, namely that the form inherits the data from its parent form.

$data = array(
    'name' => 'Foo',
    'street' => 'Bar'
);

$builder
    ->add('name', 'text')
    // e.g. a fieldset
    ->add('address', 'form', array('inherit_data' => true))
;

$builder->get('address')
    ->add('street', 'text')
;

Fieldsets can then be added to core by extending form, setting "inherit_data" to true and adding custom rendering code.

Thoughts?

Update:

BC will be kept until 2.3.