Will Northrend be affected with changes that will happen upon installing the Cataclysm expansion pack for World of Warcraft

As far as game mechanics and things like that, probably, but not much. As far as new content, quests, and other things, not really. Maybe a quest or two to send you to the new zones that will be added to the "old world", but other than that, not much should change in Northrend.