I’ve just heard that the technical paper discussing the mass and density of Phobos, as determined during the 2008 flyby, has been accepted by Geophysical Research Letters. The abstract is:
We report independent results from two subgroups of the Mars Express Radio Science (MaRS) team who independently analyzed Mars Express (MEX) radio tracking data for the purpose of determining consistently the gravitational attraction of the moon Phobos on the MEX spacecraft, and hence the mass of Phobos. New values for the gravitational parameter (GM=0.7127 ± 0.0021 x 10-³ km³/s²) and density of Phobos (1876 ± 20 kg/m³) provide meaningful new constraints on the corresponding range of the body's porosity (30% ± 5%), provide a basis for improved interpretation of the internal structure. We conclude that the interior of Phobos likely contains large voids. When applied to various hypotheses bearing on the origin of Phobos, these results are inconsistent with the proposition that Phobos is a captured asteroid.
If you are a subscriber to the journal, you can access the full paper here.
For a less technical treatment, see the piece I wrote for New Scientist, where some of the authors talk about this work in the section under the 'Space oddity' heading.
(Click on 'Full story' for full GRL journal reference).
The full reference in GRL will be:
Andert, T. P., P. Rosenblatt, M. Pätzold, B. Hausler, V. Dehant, G.L. Tyler, and J. C. Marty (2010), Precise Mass Determination and the Nature of Phobos, Geophys. Res. Lett., doi:10.1029/2009GL041829, in press. (accepted 22 March 2010)