I am attempting to overwrite a core Wordpress function (present in media.php) using add_filter() during my theme's functions.php file. This will work accord towards the blogs that I have run into, but for whatever reason I see no change whatsoever.

#override what WP is trying to do to scaling images by default
function my_image_hwstring($width, $height) {
    $out = '';
    #if ($width)
    #   $out .= 'width="'.intval($width).'" ';
    #if ($height)
    #   $out .= 'height="'.intval($height).'" ';
    return $out;
add_filter('image_hwstring', 'my_image_hwstring', 1, 2);

I favor not to edit media.php by hand because any Wordpress upgrades later will overwrite the alterations we have made. The way in which I am doing the work is correct (I believe) but is not being employed as expected.

Totally hung on this and appreciate any reactions. This can be a strange one!

What is the title from the core function you are attempting to hook into? These may help: WordPress 2.9 - List of all hooks and filters and PHP Cross Reference (PHPXref) of: WordPress 2.9.1

Simply to obvious the apparent, you are not running any kind of cache system right? Just like a cache wordpress plugin like wordpress-cache or wordpress-super cache? If that's the case, try clearing/eliminating the cache.