I haven't found a correct problem description so far for this in the discussion boards over at Apple, but I'll update this post if I find a bug description or some workaround. However, a preliminary search yields several problems relating to iCal and event invitations, but also with event invitations on Google calendars.
iCal woes: Event invite for Google Calendar is never sent
Ok, here is another bugger that annoyed me for quite some time: I am using Google Calendar as my main method of dealing with appointments and shared calendars. I use the mostly fabulous Apple iCal for managing those dates. However, if I create an event to be shared with several people, and I try to send an invitation to those people, this doe not work. In the event properties, I enter a list of Email adresses, hit "Send" and it never gets sent. The event is marked as "waiting for answers", though. But that will never happen... So I have to fire up the Google Calendar website and resend the invitations manually. Bit annoying.