It reminds me of my reaction when I saw Reddit. "Well, this is new, but I can't imagine anyone ever doing something useful with it." I was right about the first part, right about the second, but completely wrong about the implicit assumption that things have to be useful before they'll be used.
Back to the point, though: who do you see as the target demographic of overhear.us?
I suppose when I think about work, I think of my parents, and despite being technologically capable, 1) this site would get blacklisted where they work and 2) I can't see them sitting in front of a computer at home gossiping, although they do talk about work quite a bit
I can realistically envision people saying "they banned overhear.us at work" in a few months.
Question though: What stops this from being a platform for marketing - i.e. Is there a danger that the company floods the forum with bad info? What about insider information? Are you not liable if I post that my company is tanking this quarter?
Maybe there aren't that many areas on the web that are still ripe for innovation...