I believe that all you need to do is pass defaults to get_option
function init_theme_options() {
// check to see if theme options are set
// Not sure how your options are organized but...
$defaults = array(
'opt1' => 'stuff1',
'opt2' => 'stuff2'
)
$theme_options = get_option('theme_option_name',$defaults);
}
If you want to see a very complicated version of this, look at how Twenty Eleven does it.