The APA just released new rules about the citation of electronic documents with Digital Object Identifier (DOI) numbers. You can find details on the APA website at http://www.apastyle.org/elecmedia.html as well as on the University of Maryland University College Library’s website at http://www.umuc.edu/library/guides/apa.shtml#DOI.
But, students, before you apply these new rules, check with your instructor first.