Recently a customer tried to insert a youtube iframe into a jEvent which would not work. Interestingly, inserting iframes into normal posts works as expected. The solution was a little hack which prevents filtering of the request data:
components/jevents/com_jevents/libraries/iCalEvent.php, Line 356:
instead of
$array = JRequest::get('request', JREQUEST_ALLOWHTML);

it should read
$array = JRequest::get('request', JREQUEST_ALLOWRAW);

and voila: iframes are allowed in jEvents, too.

If you are using JCE and wonder why iframes still keep disappearing, edit the JCE component and add "invalid_elements: applet,script" to advanced parameters (standard is applet,iframe,script). Do not add any quotes since JCE automatically does them for you.