Wil Wheaton points to a survey of bloggers being conducted as part of a thesis investigation at Georgetown University. It appears to be a fairly well thought out survey with a good variety of demographic and behavioral questions about blogging.
Though I have a few criticisms of the survey (the caps on the choices numbers of daily and weekly visitors are really low, and the sample she’ll get will be nowhere near random), I still think that research on blogs is worthwhile and encourage you to visit the survey.