Many educators have taught digital citizenship, under another term, for a long time (Old NETS*S referred to them as social, ethical and human issues). Rather than asking: Do we need to start teaching "digital citizenship" maybe you could ask: In what ways should digital citizenship be redefined in the emerging Web 2.0 era? How might teaching digital citizenship change?
I think it's essential that principles of digital citizenry are embedded in K12 curriculum somehow. However, how are we going to do that when teachers aren't necessarily adept digital citizens? We need to focus on the teachers!
It really does have to matter, yes. The book "Ender's Game" by Orson Scott Card was about how some teens influenced the course and the opinions of society by posting online (and it was written in the 80's!) - well isn't this happening now with blogs everywhere and with major publications having comments areas where people give feedback and Presidential debates that include YouTube entries? So what does it mean then to be able to influence vast numbers of people because of content you posted that turned "viral"? What if you didn't really mean it -but now it's "out there"? How are reputations made and lost online? What does consideration and kindness mean in the online world? When the Internet has no expiration date and anything ever posted can be retrieved, even when it's deleted the next day, what do we teach our students about citizenship? Yes, we have to do this, and yes, it matters greatly.
What I love about your comparison with Ender's Game (wasn't it Ender and his sister who did all the influential posting?) is that the online dialog was pretty intellectual. I think much of what passes for youth contributions is mindless nonsense, but that means all the more for those who figure out how to say or do something in a unique and valuable way. What I worry/wonder about is how the adults seemed to have largely abdicated the responsibility for setting standards.