"I'm finished." is also something you would say to your butler when you're done eating and wants him to take the food tray away. The quasi jovial tone would suggest that it is part of the meaning as well. It's almost like an attempt at humor or sarcasm on Plainview's part as he realizes the gravity of the situation -- like it took him by surprise: "Oh, this is how it ends..." It's a final act of defiance towards life, the god he doesn't believe in and the realization that his relentless and immoral quest for power finally caught up with him. But under the subtext and heavy breath lies the truth. He knows that this is the end of the line and he finally has to accept what's coming to him.

Teddy, good point about the butler. Really, when you think about it, it's the perfect line to end the movie. BTW did you hear about the sneak of The Master at The Aero in Santa Monica this weekend? Rave reviews per tweets from people who saw it.

Yup, I read about it. Wish I'd been there. Hopefully we'll get to see it in 70mm at other screenings as per PT Anderson's wishes. I'm looking forward to seeing Joaquin Phoenix back on the silver screen, too, not just at Whole Foods shopping for veggies. I'm glad he came to his senses and got back into acting again.