Make sure this is in the head tag to make sure that phones and mobile devices are looking at your css media queries!
<meta name="viewport"content="width=device-width, initial-scale=1.0">
NOTE: For Drupal 7, since the html <head> tag is within the html.tpl.php file you can accomplish this one of two ways:
1) In your theme's template.php file, add some code for the preprocess_html function. Read here for how to add the viewport to the head tag using template.php.
2) Make a copy of html.tpl.php and put it in your theme folder. Modify it to have the view port.