Yeah. There's a lot of players in the tech world that lurk on this site, even if they don't post. I mean, Arrington, D Shah, P Buchheit and obviously PG post fairly frequently. I'm sure that O'Reilly has someone lurking here, if not reading Hacker news himself. I have no doubt that a number of angel investors and VC's read up on HN users just to find out what's bubbling among the next group of founders.
Paul early on said that one of the reasons that he made the site was to be able to get to know founders and applicants and have a bit of reference material before they apply. I'd guess it has at least a little bearing on their YC application screening. I could be wrong.
All that to say, is... Post intelligent, well reasoned comments and interesting articles. Avoid trolls and flame-baiting. Avoid posting things that you might regret later. People are reading what you post.