The nice thing about solutions like the ATEM is they have screen buffers and can synchronize non-genlocked source so that when you switch sources it's smooth and there is no glitch/roll/flicker. That's part of what that high price gets you. And oh my, Black Magic has matured the ATEM product line since I last looked - wow!
I use video matrix switchers from NTI:
http://networktechinc.com/video-matrix.html
They are significantly cheaper and can be controlled over serial or TCP/IP - I have scripts that telnet into mine and change scenes. If your sources all aren't at black before switching, you will see a glitch/roll/flicker when switching as the signals re-sync.
If you have budget for ATEM, another solution that may be better is a virtual video matrix solution from a company called Just Add Power:
http://justaddpower.com
What's unique about their solution is they somehow got an HDMI license that allows them to keep the HDMI copy protection between each source and it's transmitter, and each receiver and it's output device. This means that they can seamlessly switch between HDMI feeds - even those from copy protected sources such as a BlueRay player. That's not trivial! Hot stuff, but can get a little pricing if you have lots of inputs or outputs.
And it's fully scriptable too.
For software solutions,
Renewed Vision has some cool options that are perfect for presentations. They started providing software for churches but have really grown in the last couple of years (a scoreboard app? I wouldn't have seen that coming!). Both ProPresenter and Pro Video Player can send and receive all kinds of control signals from MIDI, DMX and others. ProPresenter DMX module:
https://www.renewedvision.com/propresen ... ons_module ProVideo Player has similar functionality.
If your cameras have firewire out, you could use either as switching software - PVP would be more suited to that than ProPresenter.
The software is not inexpensive, but it is full featured. PVP version 3 at NAB:
https://www.youtube.com/watch?v=OOpVl7SyWVo
Some pretty amazing stuff. Around the 9 minute mark he talks about multiple inputs and outputs so PVP could be a switcher now. Brad is the founder of Renewed Vision and used to be the primary programmer too.