Steve Gibson at Shacknews posted a personal tidbit about Halo as part of an Xbox launch coverage roundup:

I actually took a trip down to the local store the other day and played some more Halo. Making a couple other little kids cry by hogging the machine wasnt so bad... Anyways I dug it. It kept my interest and I only wanted to throw the controller about 3 times. Not bad for a FPS on a console. At least in the beginning of the game it seems pretty heavily story driven which was cool. No idea if that lasts through the entire game or not.

From other indications, though, I'd say that story focus does last that long...