I think it's a sensitivity to the stainless steel itself...when I used the spoon this last time, I was very careful to open my mouth wide and avoid that vulnerable corner of my mouth. There's a possibility that opening my mouth wider to accommodate the spoon makes that part of my lip crack, but since this only happens right after using a stainless steel soup spoon, I'm tending to think it's the material in the spoon...maybe there are little nicks in the steel that are catching on the lip. It's interesting that I've never had a cut on the right side of my lip, only on the left.

(And I don't know if it happens with silver spoons, because I rarely ever use real silver - because I don't like the metalic taste of real silver cutlery.