New install of WP (3.5.1) running the Twentytwelve theme. I'm not sure, but I don't think the responsive menu is acting as it should. On full screen, I get the links as links in a horizontal menu, as expected. However, if I shrink the window down, at about 600 px the nav turns into a button (still as expected). However, on my laptop, clicking on that button does nothing... there is no resulting menu for me to select from.
I do not have a mobile device other than an iPad to test on (it shows as text on the iPad) so I don't know if the issue is that the button doesn't work on a laptop or if there is something wrong with the code, or if I have munged it inadvertently. I have created a 'custom' menu so I could manage drop-downs (the theme doesn't seem to work 'out of the box' as far as drop-downs is concerned)
You can see it here: http://www.h2only.ca/wordpress/