I just got back.
First off, I had no problem whatsoever spotting PFT. For those who missed him, he's sitting to Plainview's right during his first speech. ("If I say I'm an oil man, you will agree...") PFT acquitted himself very well, as did Will Oldham in the role of Henry.
I liked the movie a lot, but agree with those who found the ending out-of-tune with the rest of the movie. It inspired thought and debate, though, so that's a good thing. As I was watching the movie, I was thinking, "Wow, this really is one of the best movies I've EVER seen," and then, when Plainview put the napkin over his face in the restaurant, things started to go slightly downhill. That said, I always had a great time as I was watching it, and was never bored. I even liked the last couple of scenes, but I liked them more as individual scenes rather than how they fit within the flow of the movie, if that makes sense. It was interesting and I'll be seeing it again soon.
A thought: (SPOILERS)
Someone wrote somewhere that Plainview could be impotent, but I think it goes further than that - the man had to be a eunuch. I'm aware that this is ridiculous, but bear with me. I think Plainview had probably suffered terrible abuse at the hands of (most likely) his father, abuse which may have involved some kind of genital mutilation. So - and here I leap shamelessly - Daniel's competetive nature would have to be his form of compensating. I also think that it was not only Daniel who was abused, but his siblings as well. So, at the beach, when Henry mentions getting some girls, Daniel is immediately convinced that his "brother...from another mother" is not who he claims. I don't believe Daniel was fostering suspicions before that scene. Daniel's reaction to Henry's plan, his return to the ocean and his attitude in the brothel (?) plot Daniel's transition from coaxing confirmation of his suspicions to disbelief to his resolving to kill Henry. (Ah, murder...is there anything it doesn't solve?) I honestly can't remember anything else from the film that would've tipped Daniel off to Henry's true identity. It is referenced in the film that Daniel has sympathy for abused children (More than most people, even!); he indirectly confronts the Sunday father by pulling his daughter aside at the refreshment table and reassuring her ("No more hitting") as her father looks on.
There's also the theory that some people have about the last scene - that it's a figment of Daniel's imagination. The evidence is there, but I'm tired of typing. Maybe later.