I just had a look at the latest HTML5 spec for this and it looks different than the older version:
http://www.w3.org/html/wg/drafts/html/master/forms.html#radio-button-state-(type=radio)
Constraint validation: If an element in the radio button group is required, and all of the input elements in the radio button group have a checkedness that is false, then the element is suffering from being missing.
With respect Takayuki, checkboxes don't look exclusive, so making them exclusive for some questions is confusing and unnecessary - that's what radio buttons are for.
Take the example of a gender question - radio buttons are the obvious choice, having checkboxes would be downright weird because of their non-exclusive appearance. If it's an important and required question, making one radio button option pre-selected is a recipe for unreliable results.
Thanks for an awesome plugin!