At the IETF meeting (may in Chicago as you suggest, but I think it was Oslo) we discussed errors to iCalendar/iTIP/iMIP. This error was in iMIP.
iCalendar, RFC 2445, just defines a MIME type. This is NOT the place to specify how you encapsulate the MIME type. We just define here a text/calendar MIME content type.
It is iMIP, RFC 2447, that defines how you send an iCalendar object in email. That email could be as a singleton, text/calendar MIME object or as a multipart object, such as multipart/mixed or such. This is where the requirement is absent. I suggest that we all go back and read the RFC 2447 spec. This is the one that specifies how you need to package the text/calendar object. This is where it currently only REQUIRES support for text/calendar and needs to also specify that you MUST be able to receive the text/calendar within a multipart/mixed.
I strongly disagree that this needs to be added as a requirement in iCalendar, which is only the specification of a C&S content-type.
-- Frank