Read more about the usage of this script on http://ryanfitton.co.uk/blog/implementing-an-image-picker-for-wordpress-3-5/

WordPress 3.5 Functions.php Script

<script type='text/javascript'>
  var file_frame;

	jQuery('.upload_image_button').live('click', function( event ){ // '.upload_image_button' is the ID of my button that opens the Media window
		
		formfield = jQuery(this).prev().attr('name'); // Needed for the script to send the URL to the appropriate text box
		event.preventDefault();

		if ( file_frame ) {
			file_frame.open();
			return;
		}

		file_frame = wp.media.frames.file_frame = wp.media({
			title: 'Choose a Portfolio Image',
				button: {
					text: 'Choose Image'
				},
			multiple: false  // Set to true to allow multiple files to be selected
		});

		file_frame.on( 'select', function() {
			attachment = file_frame.state().get('selection').first().toJSON();
			jQuery('#'+formfield).val(attachment.url);
		});
		
		file_frame.open();
	});
</script>