Well, I finally got the answer and I want to share it with all of you.
The document Introduction_DIP_21.PDF released by SAP on 2011-06-20 provided me with he answer.
Section 3.1.6 "Characteristics filled by standard sources" indicates that for the source "Actual Cost - Line Items" the characteristic ARBPL (work centre) is not filled. In Fact, It will only work for Easy Cost Planning.