After a bit of discussion on and off the yt-dev list, the mechanism for interpolation in smoothed covering grids and vertex centered data has changed in r1734. Previously, it would interpolate logged fields in log-space -- so, for instance, Density would be interpolated in log space between adjoining cells, rather than in linear space. At the end of the interpolation it was restored.
This may result in small changes in results for a small set of analysis types, but I believe that this change -- to interpolating exclusively in linear space in the unmodified fields -- is the correct change to make. To restore the old behavior, a derived field can be created that operates by returning only the logged fields, and then requesting smoothed or vertex centered data in that field.