Twitter Spam

Been thinking about this possibility a bit lately. Seems to me that there's a real danger of Twitter becoming a spammers paradise. There are very few safeguards in place (other than a captcha during sign up). People could easily @reply and direct message you until the service got so annoying you had to leave . . . Am sure the guys over at Twitter have given this though, and it doesn't seem to be an issue at the current size, but as with anything internet, the spammers are never far behind the trends.