(Sequel to part 1. I'm a software person recounting my experiences with a USB Bit Whacker, primarily as a way of keeping semi-organized notes.)
Using the default firmware, it's very simple to set digital output pins and control them. Connect to the serial device with screen /dev/ttyACM0.
There are a series of numbered pins, A0-A7, and B0-B7, one on each side of the chip. There are VCC and GND pins on each side as well. Pick a pin -- for example, A0.
Set pin the pin to be an output pin with: pd,a,0,0 (PD = pin direction, a,0 = pin A0, 0 = output).
Set the pin voltage with po,a,0,0 (low) and po,a,0,1 (high). You can verify the two values with a multimeter. (When A0 is high, compare A0->GND to VCC->GND. They should both be the 5V.)
You can now hook up an LED and a (e.g.) 1k resistor and control it! (Remember not to hook up an LED by itself -- its resistance is very low and you'll blow it up.) Whack them bits.
You can easily interact with the device from the computer by writing to /dev/ttyACM0. (I can't write to the device with echo(1) on my system, but simply opening a filehandle, e.g., with python, works just fine.)
I've written a few tiny python programs to control the device from the host. They're on github here.
Next steps: Investigate how hard it is to load user-written firmware onto the device from a Linux box.