What are your expectations of ads in Twitter? Expect it to be disruptive? Distracting? Detrimental? Personally, I await the evolution and further monetization efforts.
Twitter, like other services before it (hello, Pandora...), must try to make money to stave off a "cool but unprofitable" demise. The irony? Monetization efforts like advertising erode the desirability of the service for some users. It feels like Twitter is just entering the awkward adolescent stage of its business development: still cool, but getting pimples. We'll see how it looks when it's all grown up.