It appears to me that the text "except as a punishment for crime whereof the party shall have been duly convicted" can equally well be taken to refer to "slavery and involuntary servitude" and to "involuntary servitude" alone. Historically this exception appears to have been used in order to allow prison work, which surely falls under the rubric of "involuntary servitude" rather than slavery. Moreover, the Supreme Court has established many times that the amendment "abolishes slavery" in no uncertain terms. Some of the quotes are here: http://www.access.gpo.gov/congress/senate/constitution/amdt13.html Therefore, I think it justified to simply say that the amendment abolishes slavery.
Comments? --AV
I hadn't thought of the other interpretation. Too bad they didn't have wikipedians around to say "but hey, it could have this second unintended meaning too." --KQ