It happened to me again today (several times in all just this week)
Twitter sent me an email about a Twitter user that has decided to follow my tweets. I take a look at the Twitter ID and don't recognize it. So, I follow the link in the email to check out the person's profile. The last few tweets from this user are shown and seem a bit ... er ... NSFW. The "Web" link under "About" goes to a porn site .... great, just what I didn't want on Twitter, SPAM!
Hopefully the guys at Twitter are already aware of this and are actively doing something to fix it. I'd hate for Twitter to go the way of ICQ