On reflection, there are so many possible internal link configurations that one line can't account for them. The code you would need is already available in Smart Backgrounds, and the new version 3.0 would make it pretty easy to apply to setting the current/notCurrent class. You can find the zip in this thread until it is submitted to Free Addons.

It will require that you save an image file named according to a system for each page or category you want to mark (the content of the file doesn't matter).
The smart_backgrounds() function will return a value of '' (nothing) if an image corresponding to the current page is not found.

or (smart_backgrounds() != ''))
PHP Code:
      $page_query_list_header[$rows]['link'] .= (($current_page_base == 'page' and $page_query_list_header[$rows]['id'] == $_GET['id']) or (smart_backgrounds() != ''))? ' class="current"': ' class="notCurrent"';//gjh42 20091203 
The one limitation of this method is that if you highlight a link to a category, any subcategory or product page under that category will also read as "current".