Outlook 2003 (sp1) and Exchange 2003
The request does arrive in the inbox of the invitee.
What do you what does happen?

I make a request for several colleges. I sent the invitation. The
invitation is not read by the invitee but did arrive in there inbox.
One hour later someone else wants to plan a meeting on the same time
with the some of the same colleges. The planner does show a free
timeslot for all the invitee. So my request is not put in the calendar
of the invited colleges, not even as tentative.