I'm going to continue talking to myself as I figure this out, but I am pretty sure the problem lies in the mime header:
Working invite:
Content-Type: text/calendar; charset=utf-8; method=REQUEST; name="invite.ics"
Content-Disposition: attachment; filename="invite.ics"
Content-Transfer-Encoding: base64
X-Attachment-Id: 35a4ed2263c9cc2b_0.1
Not working invite:
Content-Type: text/calendar; name="ical-type-2.ics"
Content-Description: ical-type-2.ics
Content-Disposition: attachment; filename="ical-type-2.ics"; size=1749;
creation-date="Wed, 16 May 2012 18:35:46 GMT";
modification-date="Wed, 16 May 2012 18:35:46 GMT"
Content-ID: <calendar_message>
Content-Transfer-Encoding: base64