Is a Weblog a Business?

Clinko asks: "I currently have a couple moderately successful websites with Google and Yahoo ads. Combined, they will generate a couple thousand this year. (Sounds great, but read on...) The problem is that I know nothing about starting a business, filing taxes (U.S.) on such, or if I even need to be a licensed business. Will I need to start a licensed business on income generated from ads? I'm sure someone from Slashdot has been in a similar situation. What was your solution?"