Hello. Did you ever find a solution to this issue? I'm having the same one. My wife's iCal invitations are showing up in my iCal "Inbox" but are not coming to me via email. I discovered this while troubleshooting the issue just now...and saw that I had 10+ iCal invites for which I never received email notifications.
I never proactively check iCal, so definitely need to resolve this issue, as receiving emailed invitations is the only way they're filed into my iCal. Thanks for any insight!