When he says this line "I am honored and grateful that you have invited me to your daughter's wedding." it sounds like he thought he was not going to get invited, even though he is a cruical part of the Corleone family. I thought he was a shoe in to get an invite no matter what happened. Thoughts?

I think he just wanted to show that he was grateful to be invited. Luca was like a dumb dog... loyal to a fault, but obviously not the smartest of the bunch.

Tom says that Luca "didn't expect to be invited." In addition to being loyal to a fault (as DBCH said), Luca also recognized that he was, as Michael put it, "a very scary guy." Luca probably thought Vito wouldn't invite him because he'd scare the other guests and be a reminder that a good part of Vito's strength rested on his thuggish person.

If I may expand on what you said, TB, I think that Luca was a part of Vito's life that even HE didn't like to think about. I think that we all prefer to think of Vito as the kindly Godfather who helps out poor little bakers by twisting the arms of corrupt politicians. We don't like to think of him as the man who would kill bandleaders, not to mention rather valuable horses.

I believe that Vito liked to keep that violent side of his business hidden from his family. Luca was part of that side, and I think he was shocked to be invited into the Don's home.

SB, I think you are right about keeping it away from his family. What comes to mind is when he was younger and he was at the table with Clemenza and Tessio, and he doesn't say anything about what he is going to do. He keeps all of it to himself. In a way it is kind of ironic the way Vito was with violence and the way Micheal was, it seemed to surround Micheal even to the end.

I would have to agree with this, think back to the scene where Tom announces Brasi's wish to thank the Godfather personally - The Don responds hesitantly "...Is it neccessary?"
Tom: "He didn't expect to be invited."