Well, best not let perfect be the enemy of good. While the Freescale board is clearly better, it doesn't actually work. So here is my new Dagu controller board being run from an Arduino. The speed sensor seems to work too - much less jitter than before.
Also, top tip. Don't try to run your DCC railway with this board - it will catch fire. I don't think it like 10,000 direction changes a second.