My copy looked like this..the only good place to sign it,is in the inside insert..(as in won't wear off) case is covered with plastic..even a good sharpy would rub off sooner,or later..was shrink wrapped,so that would need to be torn off as well.. sry about the bad pictures.took them with my mp3 player.
yeah "insert" the piece of paper w/ steam key.. on a side note.. if you're gonna shelve it,and not actually play it..a sharpy on the cellophane,should work fine,but if you plan on handling it.. it needs to be signed on paper IMO.