nbme self assessments website is down so when I was looking around the faq this is what I found:
"USMLE weights all multiple choice questions equally within each Step exam. Thus, answering relatively easy questions or relatively difficult questions correctly provides equal progress toward meeting the minimum passing score. This urban myth may derive from a misunderstanding about the statistical methods called equating that ensure that the 3-digit score is comparable regardless of what test form or what time of year a candidate tests. The statistical processes make small adjustments to scores achieved on test forms that contain relatively more or less difficult items. Scores on tests with relatively difficult items are adjusted up and those achieved on relatively easy items are adjusted down. These adjustments ensure that the scores that are awarded are comparable regardless of the particular combination of items on any examinee's test form and ensures fairness for all test takers."
So I guess what that is saying is that each individual question is worth the same, but how much each is worth is dependent on the overall difficulty of the test.
source:
http://www.nbme.org/students/UrbanLegends/index.html
