Did you ever find a solution to this problem?
I too have a shared calendar with my wife and every time she adds something to the calendar, I get alerts on all my devices. I haven't found a method for turning this off and the deluge of notifications both in Calendar and in the Notification Center is driving me a bit nuts.
I think I found the solution, at least for iOS 6 devices. Under Settings->Mail, Contacts, Calendars, way at the bottom, after what looks like the end of all of the "Calendar" settings is one all by itself called "Shared Calendar Alerts" which was on. After setting it to off I'm no longer getting alerts each time my wife makes a change on one of her calendars that she shares with me.
This is with iOS 6. I don't have any devices running iOS 5 any longer so I can't check if it has the same setting.
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")