2. Phishing scams. Twitter is, of course, fertile ground for phishing scams just by the nature of how it works.
3. Worms. The Twitter worm that has received the most attention was this year's Mikeyy worm. There have been others.
4. Tiny urls. It's not easy putting your thoughts into 140 characters, especially when you have a link taking up 130 of those characters. Welcome the era of the tiny url, subsequently. This is also a great vulnerability. There are a number of plug-ins and apps you can use to test those links before you actually click through. Do so. Check out Zone Deck, Firefox's Bit.ly preview plug-in, Tweetie and Tweetdeck, as well.
5. You. Yes, you. It's so easy to tweet off at the mouth and not think about where those 140 characters are going and who will see them and, perhaps, exploit them. Be careful about giving out too much information about yourself; your comings and goings, your personal life, where you live, phone numbers, etc.