I found that if I drag the .ics attachment from mail to the me.com calendar category I want it filed under, that works as well to keep it synced.
When I receive invitations, iCal will only put them into a calendar that is "On My Mac" -- in other words, one that does not automatically sync with my iPhone and iPad. I called tech support to find out that only calendars sent to my MobileMe account can go into these calendars. That's a problem...but I have a workaround.

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