  We present the results of a program to acquire photometry for eighty-six
late-M, L, and T dwarfs using the Infrared Array Camera (IRAC) on the Spitzer
Space Telescope. We examine the behavior of these cool dwarfs in various
color-color and color-magnitude diagrams composed of near-IR and IRAC data. The
T dwarfs exhibit the most distinctive positions in these diagrams. In M_5.8
versus [5.8]-[8.0], the IRAC data for T dwarfs are not monotonic in either
magnitude or color, giving the clearest indication yet that the T dwarfs are
not a one parameter family in Teff. Because metallicity does not vary enough in
the solar neighborhood to act as the second parameter, the most likely
candidate then is gravity, which in turn translates to mass. Among objects with
similar spectral type, the range of mass suggested by our sample is about a
factor of five (~70 M_Jup to ~15 M_Jup), with the less massive objects making
up the younger members of the sample. We also find the IRAC 4.5 micron fluxes
to be lower than expected, from which we infer a stronger CO fundamental band
at ~4.67 microns. This suggests that equilibrium CH_4/CO chemistry
underestimates the abundance of CO in T dwarf atmospheres, confirming earlier
results based on M-band observations from the ground. In combining IRAC
photometry with near-IR JHK photometry and parallax data, we find the
combination of Ks, IRAC 3.6 micron, and 4.5 micron bands to provide the best
color-color discrimination for a wide range of M, L, and T dwarfs. Also
noteworthy is the M_Ks versus Ks-[4.5] relation, which shows a smooth
progression over spectral type and splits the M, L, and T types cleanly.
