Clear block cache in panels page
The variables were being cached at some point in the hook_theme() function, so after reading the documentation on hook_theme(), I moved the logic to conditionally use different variables in to my hook_block_view() implementation, passing them in to the call to theme() at that point. So that function looks like the following:

function daily_hours_block_view($delta = '') {
  if (daily_hours_is_summer()) {
    $variables = daily_hours_get_summer_vars();
  } else {
    $variables = daily_hours_get_regular_vars();
  }
  switch ($delta) {
    case 'hours_table':
      $block['content'] = theme('todays_hours_table', $variables);
      break;
  }
  return $block;
}


And my hook_theme() function:

function daily_hours_theme() {
   $items =  array(
     'todays_hours_table' =&gt; array(
       'template' =&gt; 'hours_table',
       'variables' =&gt; array(
          // set default variables here
       )
     )
   );
   return $items;
 }


The cache setting in my hook_block_info() implementation did not seem to have an impact once I set up the logic in the hook_block_view() function.