As of yet, no one of any consequence has yet to their neck out and challenge Scott Brown in Massachusetts.
The dynamics here are rather frustrating. A number of current and former Representatives in the state -- most notably Michael Capuano -- could give Brown a run for his money, if not win outright. But since Massachusetts is such a solid Democratic state, they seem to prefer to wait until Brown either gets bored or the political climate reaches a point where a Senate run would be almost 100% risk free. This sort of undue caution is a bit silly; no campaign is an absolute shoe-in, and high-quality candidates elsewhere in the country seem willing to take on significantly more risk when running for office. Why should Bay State Dems get a free shot at the seat?