I've also considered robot arms for tool changing. There are some nice options on the market, or I could just build my own with some robot servos. You don't need micron precision for a tool changer.
I actually think this would end up being more robust than the Tormach ATC, because of the positive position feedback you get from the arm servos.
There are some reasonable-cost arms available from trossen robotics / interbotix, based on Dynamixel servos. (The lowest-end Dynamixel servo AX-12 is what powers the Tormach SmartCool, btw!)