I'm not able to get dropdown to function, though I'm following along with the book (and the downloaded code here on the site) and I don't see anything incorrect about my code. I did try fiddling with the CSS like the original poster, but that attempt wasn't successful (I don't really understand what he did, and the original code should work regardless).
On page 73 of the book, when testing...
$_SESSION['user_id'] = 1;
... the "Account" <li> does display, but the dropdown will not function. Likewise, a check for $_SESSION['user_admin'] has the same result.
Otherwise, the layout looks fine.