I'm finding that the "starts on the monitor where the mouse is" thing doesn't work, at least for me. But I've got a slight variant of a secondary monitor use case. My use case is I've got 2 TV's. One is my HDTV and one is an old B+W tube TV for watching B+W TV shows and 4:3 B+W movies. So while I may be able to script moving the mouse to the other monitor when I want to switch (if that was actually working), it would be nice if there was a way to specify a monitor on startup (command line parameter, config file option, even registry option).
The way I'm doing things now with WMC is I use scripts and EventGhost to kill WMC, configure the monitor I want, and then restart WMC. So that's basically what I'd like to do with Theater (assuming there isn't a better way).
I know this is likely a very unusual use case but I'm hoping that whatever the plans are for multi-monitor support, they will accommodate my use case in some way. For now I'll probably just try using Theater on the main monitor and WMC on the secondary. My hope was to see if Theater was far enough along for me to ditch WMC altogether and upgrade to Windows 10 before the end of the free period.
As an aside, If I use shift+windows+left/right to move the window to the other monitor, the primary monitor is left with a black screen. I'm guessing that is an indication that Theater isn't currently very friendly to that method of moving the window.