The OpenSprinkler firmware can compile and run on any Linux machine (this can be done by running ‘./build.sh demo’) and we have a demo site:
demo.opensprinkler.com
which runs the firmware this way. However, if you are not using RPi, all GPIO functions are disabled (for the simple reason that a general Linux machine does not have GPIO support). You mentioned having Arduino connected to it through USB, which can certainly add GPIO interface but that’s going to require changing the firmware code, basically adding code to gpio.cpp to enable appropriate gpio support.