I've recently changed firmware v2.8 to 3.1.2, calibrated radio and accelerometer, and APM is not arming anymore. I also change ESC's to HK's Simonk, but everything else, including radio and its settings stay same. Red flashes withsingle interval and nothing changes wit stick down-right.
The board probably has crappy compass(it sends no data when enabled) but I've disabled it and this had no effect on arming APM with previous firmware.
I also know that some sort of bad TX shamanism has been required to arm early APM versions. Unfortunately, I can't remember it, and in addition to main question would be thankful if someone helped me to recall.