Ask HN: Stock Market SMS Alert Service?

I've got a new idea for a startup. I need your help.

1) What do you think: a service that lets you creates sms alerts for various stock market events? (For instance, text me if GOOG drops below X in the next Y hours.)

2) I've only found a few sites that do this now. (ex: http://www.acestockalerts.com/home/) I'm not too familiar with the stock market. Is the dearth of such services because they're not useful? Would this service in fact be useful?

3) If it would be useful to investors, what features would you recommend I start with?

Please be absolutely, painfully honest.