I know there is an official CSS3 :checked pseudo-class, but is there an :unchecked pseudo-class, and do they have the same browser support?
There is no :unchecked pseudo class however if you use the :checked pseudo class and the sibling selector you can differentiate between both states. I believe all of the latest browsers support the :checked pseudo class, you can find more info from this resource: http://www.whatstyle.net/articles/18/pretty_form_controls_with_css
Your going to get better browser support with jquery... you can use a click function to detect when the click happens and if its checked or not, then you can add a class or remove a class as necessary...