It is not true that the trade-thru rule blocks ECN executions of SPY, at prices better than the stale quotes coming from AMEX and NYSE. The SEC has, for years, granted a special, "de minimis" exemption to the trade-thru rule, allowing trade-thrus of up to 3 cents beyond the best bid or offer. The exemption is temporary, but it is expected to last until the new Regulation NMS comes into effect, which will eliminate the problem, because it will ECNs to trade thru the stale manual quotes from AMEX and NYSE.