The composite back contains nickel. My friend has a severe nickel allergy. The aluminum case was no problem, but the composite back was. (the glass) She returned it to the Apple Store within hours of wearing it, and got the stainless steel model instead. (Only the Sport model has a glass back containing nickel. Other models use zirconia.) While the case of the steel model contains nickel, she plans to use a Spigen rubberized clear plastic case which covers the metal on the back of the watch touching her skin. The only concern is that, unlike the aluminum sport, the stainless steel is prone to scratches. Any dust lodged between the plastic case and the watch will cause tiny (permanent) scratches in the highly polished stainless steel. Although it is said such scratches can be buffed out, who wants to go to all that trouble every time a scratch is noticed? It's bad enough having to constantly remove the plastic case and clean it all.
I will update on her progress.
FYI: The aluminum case does contain nickel, but it is coated, and thus doesn't cause a skin reaction.