WarCry has posted a short interview they conducted with the Warhammer Online team that attended this year's Penny Arcade Expo.
Q: Public quests are designed for a lot of players working together. Are the encounters dynamic or scalable? What happens in a few months when there aren't very many players in the newbie area? Can a player then still do a public quest?
A: Each public quest has multiple stages. The design behind them is that the first stage you should be able to solo it. So a player by himself can do the first stage. As you get into later stages of the public quests it requires more and more people, because the encounter becomes more and more difficult and complex. So you will still be able to participate and gain influence in public quests if you are by yourself, even if you can't complete it by yourself.