https://eu1.salesforce.com/help/doc/en/salesforce_spring13_release_notes.pdf, page 88
HTML Markup Escaped in Page Layout Custom Section Names
Critical Update for rendering custom section names on page layouts is
available in affected organizations. This update changes how custom
section names in page layouts are rendered, to escape HTML tags and
other markup. Prior to the Spring’13 release, custom section names in
page layouts weren’t escaped, allowing the use of HTML markup and
other tags. This wasn’t an intended feature, and the Escape Page
Layout Section Names update corrects this behavior. With this update,
tags in section names are escaped, and aren’t interpreted as markup in
the browser. For example, the left angle bracket character, “<” is
< and displays as a visible “<” in the browser.
This Critical Update only appears in existing organizations that have
markup in section names, whether for styling such as font size or
To fix them, edit the section name in the Section Properties panel,
and remove all HTML and other markup.
When you have modified any affected custom section names, you should
activate this Critical Update.