When economists try to predict the future, one of the variables they can be confident in is that Congress won't do anything. This variable is only available when there is no one group in control of decisions. If Scott Brown is elected in MA, it could be good for the whole US economy.
It isn't his pledge to vote against proposed health fixes that might change things. Instead, it is the fact tha MA will have elected a Republican to fill Ted Kennedy's seat in Congress. If MA can express that much discontent with Democrats, then other congressmen will look more closely at their own "job security". The reality of voters choosing to replace them could change votes on many things.
Of course changes late in a congressman's voting history may not be enough to save their position, but many may open their eyes. And for those who are replaced, the stronghold by one group will have been broken, and the variable that economists like to see will be in place. If they all have to try to get along, they probably will get little done. This makes predicting how their new policies and laws will affect the economy, much easier. When companies can forecast with confidence, they can make decisions.
Maybe Scott Brown Elected is a good headline for the US economy.