Horizontal scrolling in Safari on one content type only when logged out?
Looking though the markup, the problem is on the H2 for your search form.

&lt;h2 class="element-invisible"&gt;Search form&lt;/h2&gt;

Can't say exactly what's up, but if you take off the width from .level1 h2, .related h2, .tabTog h2 then it seems to be fine.