I'm a post-redditor who actively used the down/upvote feature as a disagree button. Nobody told me that this was how it was to be used because no one had to. It was a learned behavior. I followed the example set by others. Same situation when I joined Snapzu, I read how people reacted to the downvotes they got or saw others getting and realized that it was a different concept. I think others will pick up on the same vibe as long as we maintain its ideals.
Early reddit treated the downvote the same way snapzu does. When the mobile site called them "agree/disagree", people were mad. " This isn't what the downvote is for!"
But gradually the admins were seen less and less (most admins I know of are gone) and they stopped propagating that idea. I think that what finally killed it is the large digg influx that weren't told about it.
To make it sure everyone is told about it, I think an explanation pop-up on your first downvote would do nicely.
After I read my post again I probably focused this too much on confusing people, sorry. I don't think people will be not able to adapt to the culture and usage of the buttons on the site, but since it is not the same concept as "Like/Dislike" or "Helpful/Not helpful" why have it called up- and downvoting?
But I thought it through after a coffee now and as I was typing this up I realized that it is making sense as in "This is good content" and "This content does not belong here" so I guess your comments and some coffee have cleared my head enough to realize that I was probably just looking at the concept from the wrong angle. :)