There is certainly little I can add to what has already been said about the assassination of Benazir Bhutto. It was the very first news story I heard upon waking yesterday morning and I've been keeping tabs on what has been said on news sites and political blogs.
Apparently the most important thing is how this effects the upcoming Iowa caucus, at least from what you would think on reading most political sites. All the candidates, or their spokes people, appeared to take the opportunity (after briefly sending their condolences) to spin the event in a way favorable towards them or negatively towards one of their opponents.
I personally think that it very well might have been some radical element in Pakistan, not necessarily affiliated with the government. However, I also think that if (and I should underline that) the Pakistani military or intelligence services were aware of the possibility of such an incident they did little to prevent it.
I don't know enough about the entire situation in that country to even hazard a guess as to what will happen. On the other hand, I don't think you have to be an expert to foresee that things are going to get much worse and unstable in the near future. I think we can also safely bet that the Administration will blow any chance of aiding the democratic elements in that country.