It seems like it might be a good idea to know what the probability of each of these risks is.
I think you need to consider this point further. Before you go through the effort of estimating a probability it is good to know if there is any value to such an estimate. For instance, if you did a lot of work and figured out that the prob that the world would be destroyed by UFAI was 5% would that change your behavior in any way? What if you found it to be 50% or .000005%? Personally, I don't think I would do much differently. Maybe in the 50% case I would vote for the mass murder of all AI researchers but currently I don't know of any major political candidates with that in their platform. Other than that it seems like pretty useless information to me.
Here's a poser that occurred to us over the summer, and one that we couldn't really come up with any satisfactory solution to. The people who work at the Singularity Institute have a high estimate of the probability that an Unfriendly AI will destroy the world. People who work for http://nuclearrisk.org/ have a very high estimate of the probability that a nuclear war will destroy the world (by their estimates, if you are American and under 40, then nuclear war is the single most likely way in which you might die next year).
It seems like there are good reasons to take these numbers seriously, because Eliezer is probably the world expert on AI risk, and Hellman is probably the world expert on nuclear risk. However, there's a problem - Eliezer is an expert on AI risk because he believes that AI risk is a bigger risk than nuclear war. Similarly, Hellman chose to study nuclear risks and not AI risk I because he had a higher than average estimate of the threat of nuclear war.
It seems like it might be a good idea to know what the probability of each of these risks is. Is there a sensible way for these people to correct for the fact that the people studying these risks are those that have high estimate of them in the first place?