This is a fairly longstanding consensus here. It comes up every now and again though, and my position isn't the only one. Some believe up votes can mean agreement, but that obviously this has a limit. Since a down vote definitely doesn't mean disagreement, it only follows that the opposite shouldn't mean agreement. That would only serve to dilute the conversation. Like I said though, this is just a rough consensus.

Here is a previous HN discussion on comments [1].

And I'll save you the trouble of reading through to an even earlier discussion[2] where PG weighed in to say "I think it's ok to use the up and down arrows to express agreement." But this was on a different topic regarding troll-like behavior, so it's not entirely clear what his thoughts are.

But a person is going to use their up votes to mean whatever they want them to mean. If you want to up vote in agreement, go ahead, no one is going to stop you. I only up vote things I agree with if I think it adds to the conversation. So while I thought the linked video was impressive and a great way to see the product, I'm not going to up vote a comment that doesn't add much. You are free to disagree.

[1] http://news.ycombinator.com/item?id=214398

[2] http://news.ycombinator.com/item?id=117171