I’m using Oggdrop at a quality setting of 6.0, which is a nominal bitrate of 192. Just as a rough estimate what kind of bitrate would an MP3 be to achieve the same quality?

You know I actually doubt that it will cause that many arguments. All accept for some isolated samples Vorbis gives quite good results. And even though it would be a significant jump in bitrate for MP3 to provide the same over all auality we are really only talking 30 to 50Kbps more. This is of course in the midrange and up for bitrates. Say 128Kbps+. Below 128Kbps Vorbis completely blows MP3 away and even rubs WMA's nose in the mud.

From what I have seen on the Vorbis mailing list I would say that is around a 320kbps MP3, around the very high end.

And MP3Pro's too