as far as i understand iCal it is on the "receivers/subscribers" side to deal with email-updates (and not on the "senders/changers" side):
snippet from my Re: How do you stop receiving notifications about updated calendar events? (there is a screenshot as well):
to NOT get email notification about updated events from a shared calendar you subscribed to you have to uncheck "events affect availability" in that calendars "info" (mac os X.7 and X.8: iCal —> calendars dropdown —> right click on the subscribed calendar —> choose "get info" from the context-menu —> uncheck "events affect availability")