  We analyze the planetary microlensing event MOA-2010-BLG-328. The best fit
yields host and planetary masses of Mh = 0.11+/-0.01 M_{sun} and Mp =
9.2+/-2.2M_Earth, corresponding to a very late M dwarf and sub-Neptune-mass
planet, respectively. The system lies at DL = 0.81 +/- 0.10 kpc with projected
separation r = 0.92 +/- 0.16 AU. Because of the host's a-priori-unlikely close
distance, as well as the unusual nature of the system, we consider the
possibility that the microlens parallax signal, which determines the host mass
and distance, is actually due to xallarap (source orbital motion) that is being
misinterpreted as parallax. We show a result that favors the parallax solution,
even given its close host distance. We show that future high-resolution
astrometric measurements could decisively resolve the remaining ambiguity of
these solutions.
