Why doesn't google consider that introduction that you see on vBulletin sites when a user is logged out as duplicate content. It seems to be the same generic message on all vB forums and it appears on all the pages of the forum... so why does google not consider it duplicate content?
Maybe it does. Maybe it doesn't. Forums are not competing to rank for the text you are talking about so it won't matter. Google tries to return good results for its users so its not going to filter out forums like this just because they use a certain type of software.
Yeah, the little content that there is in that message is not something that google is going to index very seriously...and if it's still something you're concerned about, just remove that text or make your own