is pretty much exactly what I've been experiencing, though I'm only rocking 2 monitors. Applications on the left monitor will spawn their modal dialogs on the right monitor, hugging the left border, vertically centered relative to the Y resolution of the left monitor. As much as I've learned to live with it, it still does drive me bonkers sometimes.
Is there a way around this? I'd love it if modal dialogs would show up on the screen that my cursor was on. CompizConfig settings with the Place Window plugin don't seem to have any effect.
Running Ubuntu 11.10 x86-64, HD3000 video.