  We present preliminary trigonometric parallaxes of 184 late-T and Y dwarfs
using observations from Spitzer (143), USNO (18), NTT (14), and UKIRT (9). To
complete the 20-pc census of $\ge$T6 dwarfs, we combine these measurements with
previously published trigonometric parallaxes for an additional 44 objects and
spectrophotometric distance estimates for another 7. For these 235 objects, we
estimate temperatures, sift into five 150K-wide $T_{\rm eff}$ bins covering the
range 300-1050K, determine the completeness limit for each, and compute space
densities. To anchor the high-mass end of the brown dwarf mass spectrum, we
compile a list of early- to mid-L dwarfs within 20 pc. We run simulations using
various functional forms of the mass function passed through two different sets
of evolutionary code to compute predicted distributions in $T_{\rm eff}$. The
best fit of these predictions to our L, T, and Y observations is a simple
power-law model with $\alpha \approx 0.6$ (where $dN/dM \propto M^{-\alpha}$),
meaning that the slope of the field substellar mass function is in rough
agreement with that found for brown dwarfs in nearby star forming regions and
young clusters. Furthermore, we find that published versions of the log-normal
form do not predict the steady rise seen in the space densities from 1050K to
350K. We also find that the low-mass cutoff to formation, if one exists, is
lower than $\sim$5 $M_{Jup}$, which corroborates findings in young, nearby
moving groups and implies that extremely low-mass objects have been forming
over the lifetime of the Milky Way.
