I miss the good ol' days when this wasn't an issue. I remember one time I arrived in HNL and checked with the Hawaiian airlines counter airside about moving my seat, and handed her both my DL (F) boarding pass, and my seperate HA (Y) boarding pass and the agent said something along the lines of "Oh, you flew here in F on Delta? Let's just make it F all the way to Lihue" and upgraded me for free.

That was the same day I met the cast of Lost in the Inter-Island terminal. Someone had to tell me who they were and why people were talking to them. I had never (and still never) have seen the show.

Back on topic, the other posters are spot on due to my recent experiences. Delta DID check my bag through, saying they were going to do me a one time favor, but HA was having none of it on the return. That was a massive pain in the butt on arrival into SEA in the very early hours of the morning approaching being awake for 24 hours.