I wound my way to John Scalzi's interesting blog via Instapundit.

He talks about the Big Bang, theory vs. hypothesis, belief, science, and religion. Interesting stuff. Among other things he delineates the common-use meaning of the word "theory" with the scientific definition:

For example, the word "theory." Commonly speaking, "theory" equates to "whatever ridiculous idea that has popped into my head at this very moment" -- so people have theories about UFOs, alligators in the sewers, the Kennedy Assassination, the healing power of magnets and so on. The somewhat debased nature of the word "theory" is what allows Creationists and others to say "it's just a theory," about evolution or the Big Bang or whatever bit of science is inconvenient to them at the moment, implicitly suggesting that as such, it should be paid little regard.

However (and Astronomy magazine has a nice sidebar on this), the word "theory" means something different to scientists than it does to the average Joe. In the world of science, the initial crazy idea that you or I would call a theory is a "hypothesis"; it's not until you can provide strong, verifiable evidence that the universe actually conforms to your hypothesis that you're allowed to say it's an actual theory.

He also does a good job of defining "belief":

"Believe," incidentally, is another problem word, since its common usage is synonymous with "I have faith," and faith, by its nature, is not particularly evidentiary. Someone who says "I believe in Jesus," is declaring faith in Christ, whose nature is ineffable. One wouldn't say that one has faith in the Big Bang -- and rightly so.

Fundamentally, one doesn't "believe" or have faith in much of anything as it regards science, since as a process science isn't about believing at all. It's about testing and verifying, discarding what doesn't work, and refining what does work to make it better describe the nature of reality. For a scientist, a belief functions at the level of a hypothesis, which is to say, it's an idea that requires testing to determine whether it accurately models reality.

Although here I may slightly disagree with his assertion that people don't have faith in anything with regard to science. I would still say, for example, that I believe that the earth orbits the sun (and not vice versa).

The big difference is that people once believed the opposite to be true based on religious documentation and the word of the church, as opposed to actual testable, verifiable information.

This is a distinction that the late Isaac Asimov so eloquently made:

One person recently, goaded into desperation by the litany of unrelieved negation, burst out. "Don't you believe in anything?"

"Yes," I said. "I believe evidence. I believe observation, measurement, and reasoning, confirmed by independent observation. I'll believe anything, no matter how wild and ridiculous, if there is evidence for it. The wilder and more ridiculous something is, however, the firmer and more solid the evidence will have to be."

I once heard an anecdote with regard to Asimov that he also said something to the effect that, yes, the notions that the earth is flat and that the earth is spherical are both basically beliefs, but by virtue of evidence and reason, one is vastly more valid than the other. In other words: not all beliefs are equally valid. Some have much more merit than others, such as beliefs based on rigorous reasoning and evidence.

For example, you have good reason to believe that if you hold up a golf ball and drop it, gravity will pull it toward the earth at a constant rate of acceleration. We've got a lot of evidence for this phenomenon.

You don't have very good reason to believe in unicorns (or spirits, or astrology, or any number of other bits of paranormal hogwash). Why? Because they don't hold up to standards of reason and evidence.

So yes, scientists believe. But some beliefs are much more valid than others.