  We present Spitzer IRAC photometry of twelve very late-type T dwarfs; nine
have [3.6], [4.5], [5.8] and [8.0] photometry and three have [3.6] and [4.5]
photometry only. We investigate trends with type and color for the planning and
interpretation of surveys for the coldest T or Y dwarfs. Brown dwarfs with
effective temperature (T_eff) below 700K emit more than half their flux at
wavelengths beyond 3um, and the ratio of the mid-infrared flux to the
near-infrared flux becomes very sensitive to T_eff. The color H-[4.5] is a good
indicator of T_eff with a relatively weak dependence on metallicity and
gravity. Conversely, the colors H-K and [4.5]-[5.8] are sensitive to
metallicity and gravity. Thus near- and mid-infrared photometry provide useful
indicators of the fundamental properties of brown dwarfs, and if temperature
and gravity are known, then mass and age can be determined from evolutionary
models. There are twelve dwarfs known with H-[4.5]>3.0, and 500< T_eff K < 800,
which we examine in detail. The ages of these dwarfs range from very young
(0.1-1.0 Gyr) to relatively old (3-12 Gyr). The mass range is possibly as low
as 5 Jupiter masses to up to 70 Jupiter masses. The metallicities also span a
large range, from [m/H] -0.3 to [m/H] +0.3. The small number of T8-T9 dwarfs
found in the UKIRT Infrared Deep Sky Survey to date appear to be mostly young
low-mass dwarfs. Extensions to the mid-infrared space missions warm-Spitzer and
WISE are desirable in order to obtain the vital mid-infrared data for cold
brown dwarfs, and to discover more of these rare objects. (Abridged.)
