So this is basically a brokerage. You let your customers trade with one another and you hedge their positions on the exchange. Meanwhile, you extract a spread from all transactions.
I will pass on this product, as I am developing my own web-based trading platform that actually sends orders to the market.
That will be a good try. It seems that Bitcoinica will get competitors soon. (Haha, joking.)
I think yours is like a web-based API wrapper. That will be useful for traders who are not comfortable with other exchanges' interfaces or want more features that can be done via API.
Actually being a brokerage is just the very beginning for Bitcoinica. We have future plans too. We will keep everyone updated.
Taking this strategy makes Bitcoinica the third largest trading platform right now (3,724 BTC traded in first 24 hours). We have successfully avoided the network effect associated with every exchange. (Positive feedback loop: Thin order book -> No volume -> No traders -> Thin order book -> No volume ...)