  We present a review of precursor observing programs for the SIM PlanetQuest
Key project devoted to detecting Jupiter mass planets around young stars. In
order to ensure that the stars in the sample are free of various sources of
astrometric noise that might impede the detection of planets, we have initiated
programs to collect photometry, high contrast images, interferometric data and
radial velocities for stars in both the Northern and Southern hemispheres. We
have completed a high contrast imaging survey of target stars in Taurus and the
Pleiades and found no definitive common proper motion companions within one
arcsecond (140 AU) of the SIM targets. Our radial velocity surveys have shown
that many of the target stars in Sco-Cen are fast rotators and a few stars in
Taurus and the Pleiades may have sub-stellar companions. Interferometric data
of a few stars in Taurus show no signs of stellar or sub-stellar companions
with separations of <5 mas. The photometric survey suggests that approximately
half of the stars initially selected for this program are variable to a degree
(1 sigma>0.1 mag) that would degrade the astrometric accuracy achievable for
that star. While the precursor programs are still a work in progress, we
provide a comprehensive list of all targets ranked according to their viability
as a result of the observations taken to date. By far, the observable that
moves the most targets from the SIM-YSO program is photometric variability.
