(This would probably require both calendar and mailer work.)

It would be clever if instead of sending just text/calendar attachments
for meeting requests, if we instead did something like:

Content-Type: multipart/alternative; boundary="=-="

--=-=
Content-Type: text/plain

Dan Winship has sent you an iCalendar meeting request, which your
email client does not know how to automatically process. Here
is the information about the proposed meeting:

...

You should reply to the sender saying whether or not you are
available at this time.

--=-=
Content-Type: text/calendar; method=REQUEST

BEGIN:VCALENDAR
...
END:VCALENDAR
--=-=--