It seems to me one of the crucial factors will be how the ARCA book is displayed on the screen. Can I just enter a ticker in RealTick and get it along with NYSE and regionals? If the whole ARCA book for listed will show on a Lev II screen in realtime, that spells big trouble for NYSE, particularly their so-called open book. Goldman Sachs has an interest in ARCA, so generating enough liquidity to make it worthwhile should not be a problem.
It is just a fact of life that floor-based trading will disappear in the next few years. It's already happening in europe, the chicago commodity exchanges are fighting it but recognize it as inevitable. NYSE may be the last holdout but being able to execute against a transparent book is just too attractive.