Bush Administration and Hariri
This reaction was predictable enough that it casts serious doubt on whether Syria is responsible, as they have been seeking to cooperate with the U.S. over Iraq and general terrorism issues, as well as restart the peace process with Israel. And while no one has suggested this and I have no idea how plausible it is, I think Iran is actually gaining the most from this situation. Lebanon still has intercommunal tensions, and Hariri was a Sunni leader. The focus of American attention in the region has at least temporarily shifted from Iran to Syria. If the U.S. does become involved in a significant confrontation with Syria, it will hinder Middle East peace and perhaps weaken Abu Mazen enough to allow Iran to increase its direct influence over Palestinian militants, which regular readers know it is almost certainly doing. Furthermore, if Iran wants an Iraq that is messed up but not absolutely chaotic as some suggest, then pushing Syria away from cooperation with the U.S. against the insurgency is one way to do that. All of this is highly speculative, but so is everything at this point.
UPDATE: To quote Matthew Yglesias: "The Bush administration is quickly rushing to blame Syria for the killing, which certainly could be an accurate assessment of what happened. But then again it might not be. This is the sort of situation where it would be far, far better for the United States not to be run by a group of people who have a track record of thinking nothing of embroiling the country in military conflicts based on aggressive spinning of sketchy intelligence information." I agree.