Unfortunately, it requires a change to the core.
File: administrator/components/com_contact/admin.contact.php
Line: 265 (as at Joomla 1.5.20), which reads:
Code: $post[‘misc’] = JRequest::getVar(‘misc’, ”, ‘POST’, ‘string’, JREQUEST_ALLOWHTML);
Change JREQUEST_ALLOWHTML to JREQUEST_ALLOWRAW.