This is fantastic!
By the way, I like the trading gateway idea. This is something that I hope will eventually be implemented in BitShares directly: limit orders + atomic chained fill-or-kill market orders (basically what Ripple already does with their path finding algorithm) with price limits on the end-point asset conversion. But until that day, CFS could do it at the cost of exposing the user to counterparty risk over the <20 second duration of the trade
. I would also like to see you guys go further and actually implement the path finding algorithm on your end to provide the most optimal conversion path, allow the user to specify conversion limits so that you do not go through with the order if the best available liquidity is not good enough (as determined by the user), and even allow the user to specify a fixed amount of the end-point asset that they want delivered to another BTS account (with the remainder returned backed to the user). With these features implemented, light wallets could easily integrate an API that you guys provide to pay a fixed BitUSD payment to a merchant which is paid for by the user's BitGold balance, all within 20 seconds.