@squinney,
It's probably easier to think of them as being "collapsed." They're there, just not immediately visible.
As far as I know, currently, every single post is be visible to people who want to see them (whether it's collapsed or not). Craven has said that some illegal stuff will be removed entirely, but I think that for most practical purposes, you can assume that everything will be ultimately visible, just collapsed or not.
People can choose their settings for what is collapsed and what isn't. Default seems to be -10 for a topic, and -5 for a post.
That means that (if you have the default setting), if 10 people have voted a topic down, it's collapsed for you on any list of topics. If 5 people have voted an individual post down, it's collapsed for you on any thread.
My understanding is that once you vote something down (a topic or a post), it is collapsed for YOU, but it needs to reach the threshold for it to be collapsed for other people.
Make any sense?