Spam bot generating comments that get voted up? Reaching the right level of sophistication, these spam bots would suddenly turn into quite acceptable members of the community, even if they are completely automated. If readers in general find these comments worth up-voting, than it would not matter much what the underlying intent was, because it is not harmful anylonger.
A spam bot can find a link that is published at the same time on reddit (by example) and HN, take some comments with lots of upvote and submit-it to HN. He will probably get some upvotes.
There are actually already spambots that do this on reddit -- they find a highly-voted comment in a reddit thread, follow the link to the article and repost the comment there (filling in the URL they want if the site in question allows it). Unless you look carefully at what people claim as "their site", or happen to read reddit as well, they can be hard to catch.
