Notification box in iCal
As discussed in Bug 40541 - ical notifications box not functioning is this working for anyone?
I was told it would work in 5.0.19 but upgrading didn't help.
Note that the notification box does kind of work if you have a local calendar ("On My Mac"). But I don't want my users to have local calendars--it would just lead to confusion as they put meetings in a place that doesn't sync with the server or affect their free/busy.
I haven't run into this problem but that's because our stance is to allow local calendars. Why don't you just ..not worry about them mis-filing it? You can't eliminate all PEBKAC situations. Then again I guess you want to minimize it.