Is it allowable to take courses toward the concentration for "S"? And,is the decison permanent once made?
YES. There is actually no restriction for the use of the "S" option, concentration or not. However, one should employ good judgment in making the decision, since future goals (eg, pre-med, etc.) may make it preferable to stick with grades for certain key subjects, for the most part.
Also, note that the "S" is NOT a screen to hide "C," but was introduced to encourage judicious academic risk taking and exploration of areas of study outside your personal comfort zone.
YES, the grade option selected as of a certain date each semester, is PERMANENT, and cannot be changed back. That date coincides with the last day to add a class, with a late fee. For the Fall 2012, that date is Oct. 4.