Actually, the HTML 4.01 spec says that these attributes do not require values. I haven't personally encountered a situation where providing a value rendered these controls as unselected.
Here are the respective links to the spec document for selected and checked.
Edit: Firebug renders the checkbox as checked regardless of any values I put in quotes for the checked attribute (including just typing "checked" with no values whatsoever), and IE 8's Developer Tools forces checked="checked". I'm not sure if any similar tools exist for other browsers that might alter the rendered state of a checkbox, however.