If you try to insert text talking about HTML into a rich text field, the < goes away. E.g. in a field that uses FCK, insert the text <P>. When you open the field for editing, it will have turned into the tag <p>.
In .vm files that call FCK, escapeHTML is called. FCK assumes that < appears as < If it appears as <, it is taken as part of a tag, if the text after it is a legal tag.
I believe the solution is in formFieldTemplate.xslt. In the richText-field template, the textarea is set up using
<xsl:value-of select="$currentNode" disable-output-escaping="yes" />
I believe you want disable-output-escaping="no"