  We present a census of circumstellar disks in the Chamaeleon I star-forming
region. Using the Infrared Array Camera and the Multiband Imaging Photometer
onboard the Spitzer Space Telescope, we have obtained images of Chamaeleon I at
3.6, 4.5, 5.8, 8.0, and 24 um. To search for new disk-bearing members of the
cluster, we have performed spectroscopy on objects that have red colors in
these data. Through this work, we have discovered four new members of
Chamaeleon I with spectral types of M4, M6, M7.5, and L0. The first three
objects are highly embedded (A_J~5) and reside near known protostars,
indicating that they may be among the youngest low-mass sources in the cluster
(<1 Myr). The L0 source is the coolest known member of Chamaeleon I. Its
luminosity implies a mass of 0.004-0.01 M_sun, making it the least massive
brown dwarf for which a circumstellar disk has been reliably detected. To
characterize the disk population in Chamaeleon I, we have classified the
infrared spectral energy distributions of the 203 known members that are
encompassed by the Spitzer images. Through these classifications, we find that
the disk fraction in Chamaeleon I is roughly constant at ~50% from 0.01 to 0.3
M_sun. These data are similar to the disk fraction of IC 348, which is a denser
cluster at the same age as Chamaeleon I. However, the disk fraction at M>1
M_sun is significantly higher in Chamaeleon I than in IC 348 (65% vs. 20%),
indicating longer disk lifetimes in Chamaeleon I for this mass range. Thus,
low-density star-forming regions like Chamaeleon I may offer more time for
planet formation around solar-type stars than denser clusters.
