Hi Murat,
Yeah, the idea of running AAA and Direct Booking at the same time is what bothers me (it's what we're doing now to keep the "already booked" message).
The main thing that bugs me is that Direct Booking means I need to give users write-access to the resource calendars. This means that often users will have a room's calendar open in Outlook to see if it's busy, and then click the "New Meeting Request" button, which will actually create the meeting *as the room* rather than as themselves. We've had this happen quite a bit.
So I guess I'm hoping that in a future version of Exchange there will be the ability to show the "already booked" message without requiring Direct Booking.
Matt