  We report a Spitzer/IRAC search for infrared excesses around white dwarfs,
including 14 newly-observed targets and 16 unpublished archived stars. We find
a substantial infrared excess around two warm white dwarfs ---
J220934.84+122336.5 and WD 0843+516, the latter apparently being the hottest
white dwarf known to display a close-in dust disk. Extending previous studies,
we find that the fraction of white dwarfs with dust disks increases as the
star's temperature increases; for stars cooler than 10,000 K, even the most
heavily polluted ones do not have ~1000 K dust. There is tentative evidence
that the dust disk occurrence is correlated with the volatility of the accreted
material. In the Appendix, we modify a previous analysis to show that
Poynting-Robertson drag might play an important role in transferring materials
from a dust disk into a white dwarf's atmosphere.
