According to developers Bungie, Halo 3's single player game is finished, "end to end."
"Pretty much every level is not only in the build, and for want of a better word, populated, but that the levels actually flow together pretty well. It's playable enough now, from end to end, that we've started testing properly for difficulty and length. Performance is all over the place of course, from build to build," Bungie's Frank O' Connor told IGN.
There's still a lot of work to be done on the game's graphics, playability, voice-acting and a billion other details, but I'm sure everyone involved is working hard. Except maybe one guy who hangs around the office drinking coffee and talking about 24.