Saul Hansell of the New York Times tweets that he's testing just such a system. Now, it's possible that the tech writer was trying out a third-party advertising platform; i.e. ads served by a company other than Twitter Inc.
No matter: The concept is sound, and contextual ads based on user input have been Google's cash cow; given how many of its users tweet in order to find information, Twitter would be wise to at least test out such an elegantly simple system, if the microblogging service can find a way to show the text ads unobtrusively (for example in the sidebar, where it places those paid concept definitions).
As far as conceivable multimillion-dollar advertising schemes go, that would be among the least obnoxious. And if there's one thing Twitter users hate, it's obnoxiousness, right?? (*Cough*)
(Top pic: Twitter CEO Evan Williams at the company's San Francisco HQ, March 10. Getty Images.)