I would like to spread my a camera view over 3 monitors, in full screen mode, on a mac, with another view on a fourth.
Since fullscreen mode only wants to use one monitor, I have been pondering running 3 instances with 3 cameras, and syncing them with networking…this allows me to compensate for the angle of the monitors. as well as their edges.
Unfortunately, each instance will only go fullscreen in the middle monitor. Does anyone know of a way I can specify which monitor an instance runs in?
It’s not possible at this time, I don’t think.
I know multiple monitor support has been feature-requested before, so you might search for that thread and bump it. I really want it too 
Actually, it is possible using a special triple screen adapter from Matrox called a “TripleHead2Go” ( http://forum.unity3d.com/viewtopic.php?t=7378&highlight=matrox ). At least on a Mac and according to Bjerre who posted a photo and explanation about halfway down the first page of that thread. As of Unity version 2.1, it should be possible under Windows also IIRC. I had requested this capability and I believe Aras added this in the last update. I still haven’t had an opportunity to test it though.
That triplehead2go looks like it requires hardware…is that the case?
Yes, the TripleHead2Go -is- a hardware unit. There should be more info on the Matrox site if you need it. I don’t have one of these, nor have I seen one, but the poster in that thread I listed has apparently gotten it to work.