Unless you have inside knowledge or an unfair advantage is perfectly ethical. Assuming speculating is a zero sum game (so for every winner there is a loser) it is perfectly ethical
If you play tennis with a friend and you beat him are you being unethical?
On another note you could say traders are keeping price fair by speculating in it. A company that fiddles its books is trading at a yearly high, is it ethical that this company and its shareholders are being financially rewarded by lying about their profits? Of course not, so traders short the stock and bring it down to a fair value that represents what the company is worth. Just as a company that works hard and is successful gets rewarded by speculators buying in to it.
Competition is what causes an increase in standards, all companies try to provide the best service they can to win our faith (in terms of financial investment), they get financially rewarded, we get better services, hence why communism doesnt work, because rewards are capped so why bother working harder. If the services are substandard and not worthy of what they are being paid we bring its value down.
This is very ethical in my view.
But on another note, dont worry about it, if you contemplaint the moral consequences of every action you take you would go insane, is breathing moral? After all you are taking o2 from the people around you and producing co2 which they cant use. why not rob them while youre at it! ;-)