Someone with this attitude would have a definite or even strong opinion on a given issue, but would also be willing to change their mind should new information arrive. The word could also be used to describe someone who was open minded, but also had a clear working opinion which they were prepared to use with confidence.
An example of this would be an Agnostic who nevertheless considers it very unlikely that God exists. But the meaning shouldn't be limited to a religious context.
Agnostic is close, but only seems to describe the open minded aspect of the attitude, not the having a definite working opinion aspect.