Solution 1:

jQuery('#main').css('opacity') = '0.6';

should be

jQuery('#main').css('opacity', '0.6');

Update:

http://jsfiddle.net/GegMk/ if you type in the text box. Click away, the opacity changes.

Solution 2:

Try with this :

jQuery('#main').css({ opacity: 0.6 });

Solution 3:

Try this:

jQuery('#main').css('opacity', '0.6');

or

jQuery('#main').css({'filter':'alpha(opacity=60)', 'zoom':'1', 'opacity':'0.6'});

if you want to support IE7, IE8 and so on.