News Editors
SE now supports submitting news in Markdown and XHTML along with the existing WYSIWYG editor.
That means you should theoretically be able to get MathML going in news items. However, I haven't been able to myself. I really don't want to, and shouldn't have to, change the doctype of the entire page to insert a MathML fragment, especially given that's what namespaces are for, no?