My GF flew SEA to MSP this past Sunday and back Monday, while I did a same day run to PHX on the Monday. She used NW's web checkin and I used AS's ITM check in.

She was never asked for her I.D. nor was I. I did offer my I.D. to the AS gate agent in PHX and to the gate agent's credit, he did look at it.

Of course not that proper I.D. mattered for Tuesday.