I emailed them and asked if my space black stainless steel would scratch on the aluminum body and this was their response..
"Your question is a valid concern with the recent reports of scratching found on Apples 316L stainless steel cases. During initial design, our engineers took the material hardness into account between the two adjacent materials, 316L stainless steel and anodized 6061-T6 aluminum. Weve also taken additional measures with in-house testing of 316L stainless steel against anodized aluminum and found there was no scratching. With normal intended use, this wont be a concern."
Does that sound reassuring enough?