There seems to be a theme running around the intertubes today... a them about skepticism. I'm nothing if not unoriginal, so here are my thoughts on the matter.
Skepticism is not saying everything is wrong. Skepticism is saying that you aren't sure of something without further evidence.
Skepticism is not making up arguments to support your thoughts on the subject (pro or con). Skepticism is honestly trying to find the information that you need to stop being skeptical.
In that way, skepticism should be about data, logic, and the scientific method. Every scientist should be a skeptic. But skepticism does not mean doggedly defending your point of you beyond the realm of rationalism either.
Many people claim to be skeptics. Some of them even are. Most are not (about something). We all have our pet notions. "If I wear my lucky socks on game day, then the Cowboys will win."
Skeptics can be convinced by sufficient information. That's one way in which evolution-deniers and climate-deniers are not skeptics. They cannot be swayed by sufficient information.
Muller is a skeptic. He thought climate scientists were wrong. He spent two years looking at the data... not to prove the climate scientists wrong, but to find the correct answer to the question he thought was important. He got the results of his work and is now convinced that the Earth is warming. I think he remains skeptical about human causation of the warming trend, but maybe he will continue his research.
Watts is not a skeptic. He is a denialist. He said he would change his mind and accept whatever Muller found, but he has not.
I could go on and on.
The point is that a skeptic can change his/her mind to fit the evidence. Does that mean a skeptic must have every possible answer complete? No. If every possible question was answered, then scientists would be out of a job. It would all just be making license plates from then on.
At what point can a skeptic make a decision one way or another, well, that all depends on the question, the evidence, and the skeptic. As Carl Sagan said, "extraordinary claims require extraordinary evidence".
For Muller, it was 2 years of heavy data analysis.