A few months ago I said that it's not possible to spam Twitter. This is because you can choose who you follow. And people can't send messages to you if you don't follow them. This makes Twitter unspammable.
The above is still true, with the exception that with search.twitter.com people will find messages of people that they don't follow. This problems becomes bigger when people will start searching for their @usernames. If someone now tweets with a message containing your username it will appear in your client.
Today I say another way to make the same thing happen. Twitter generates a list of trending topics in the sidebar. These topics are links to the search engine. By clicking one of these links you will get a list of all tweets containing these search terms. The way to take advantage of this is to pick a word from the trending topics and write a tweet with those words. It will now appear in front of a lot of people.
Also see Mashable.