Twitter is fast-encroaching on your web freedoms! (Kidding, you have none.) The open forum for intelligent tweeting will begin a personalized advertising system based on users' interests.
The same way that "becoming a fan" of Vampire Weekend on Facebook will probably land you a Neighborhoodies ad, or the way that being a female on Facebook will land you an Oprah Acai Berry Diet ad, Twitter also wants to aggregate your interests in a creepy and knowing way. But how do you aggregate information on a person based on tweeting? Is there an algorithm for "i just ate all the sandwiches lol" ?
Nope — the ads will be based on who you follow. If you follow The Onion, you'll probably see some Onion ads (those guys have their finger in every pie these days). If you follow Tracy Morgan, you'll probably get ads from NBC. If you follow everyone that follows you, you will probably see a lot of dating ads for adult websites. Get ready for your Twitter sidebar to annoy the shit out of you.