A warning to everyone out there! There are many websites selling traffic in various quantities. One I tried once was offering 1 million unique visitors for $30. The traffic was generated very quickly, and I even realized a small increase in my Google earnings until I received a letter from Google claiming that I was getting fake traffic and that if I didn't cease immediately my account would be terminated. Luckily I was able to cancel the add program before I lost my account.

I did write back to Google and explain the situation and that I wasn't aware that the traffic program was going to use robots (it claimed it was all human traffic), and I did get a human response explaining that most "pay for traffic" schemes either use robots to generate at least part of the traffic. As soon as any robots visit your site and start clicking around a red flag goes up and a letter is sent. If it continues beyond a couple days after the communication is send, the account is terminated.